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#. our project is no. 4 in the list . 
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4. Suppose that you roll four 6-sided dice. Estimate the probability that the die rolls are such that the four dice can be paired off so that one pair sums to 7 and the other sums to 9.
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becasuse the condition is paired dies' sum should be 7 or 9 ,so the choices for 2 chooses from 4 will be 4*3/2 =6 , that means there will be six different appearances that satisfy the requests(+1). besides these 6 phases, others are all no useful(+0).
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in order to make it more accurate, we did two ways  as follows and we have similar solution which is arround 0.075 which should be correct answer. 
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solution 1 : 
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times=[]                                              # int the trials that fit for the condition
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import numpy as np
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for e in range(0,1000000):                            # input the trials we want
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    res = np.random.randint(1, 7, size=1)             # die 1 : the random number on 6 sides die roll once 
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    res2 =np.random.randint(1, 7, size=1)             # die 2 : the random number on 6 sides die roll once 
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    res3= np.random.randint(1,7,size=1)               # die 3 : the random number on 6 sides die roll once
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    res4=np.random.randint(1,7,size=1)                # die 4 : the random number on 6 sides die roll once 
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    if res +res2 == 7 and res3+res4 == 9:             # one posibility if the sum of no1 and no2 could be 7 and others two is 9, then add 1 
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        i=1
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    elif res+res3==7 and res2+res4==9:                # one posibility if the sum of no1 and no3 could be 7 and others two is 9, then add 1 
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        i=1
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    elif res+res4==7 and res2+res3==9:                # one posibility if the sum of no1 and no4 could be 7 and others two is 9, then add 1 
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        i=1
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    elif res2+res3==7 and res+res4==9:                # one posibility if the sum of no2 and no3 could be 7 and others two is 9, then add 1 
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        i=1
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    elif res2+res4==7 and res+res3==9:                # one posibility if the sum of no2 and no4 could be 7 and others two is 9, then add 1 
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        i=1
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    elif res3+res4==7 and res+res2==9:                # one posibility if the sum of no3 and no4 could be 7 and others two is 9, then add 1 
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        i=1
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    else:                                             # different from these 6 choices are unfit for the condition, then add 0 
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        i=0
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    times.append(i)                                   # keep trying the trials untill 1000000 times
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pro=np.mean(times)                                    # def probability is mean of all "1"
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print(pro)
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the output :  0.074081
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solution 2 : 
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def die_rolls():                     # defining die_roll for multiple runs
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    D1= np.random.randint(1,7)       # die 1 : random number on a six number die
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    D2=np.random.randint(1,7)        # die 2 : random number on a six number die
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    D3=np.random.randint(1,7)        # die 3 : random number on a six number die
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    D4=np.random.randint(1,7)        # die 4 : random number on a six number die
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    if (D1+D2==7 and D3+D4==9) or (D1+D2 ==9 and D3+D4==7): # This is one of the possible result we want where
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        return 1                                            # D1 & D2 is a pair and D3 and D4 is a pair. and one pair
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                                                            # adds up to 7 and the other adds up to 9. Which we return 1
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    elif (D2+D3==7 and D1+D4==9) or (D2+D3 ==9 and D1+D4==7):# This is one of the possible result we want where
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        return 1                                             # D2 & D3 is a pair and D1 and D4 is a pair. and one pair
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                                                             # adds up to 7 and the other adds up to 9. Which we return 1
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    elif (D2+D4==7 and D1+D3==9) or (D2+D4 ==9 and D1+D3==7):# This is one of the possible result we want where
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        return 1                                             # D2 & D4 is a pair and D1 and D3 is a pair. and one pair
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                                                             # adds up to 7 and the other adds up to 9. Which we return 1
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    else:                                                    # (Unwanted result) None of the statement above is true /
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        return 0                                             # Isn't a result where one pair adds up to 7 and the other
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                                                             # pair adds up to 9. So it returns 0
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  sum([die_rolls() for i in range(1000000)])/1000000             # adds up all the '1's (wanted results) and divide it by the
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                                                            # total number of trials. (in this case 1000000)
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the output :          0.073664
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