Answer key for Math question (2)

a) 7C4 [ (0.9)^4 * (1 - 0.9)^(7 - 4) ] 
     = 7!/(4!*3!) [(0.9)^4 * (0.1)^3] 
     = 35(0.0006561) 
        =0.023

b) 7C4 [p^4 * (1 - p)^3] 
     = 35p^4(1 - 3p + 3p^2 - p^3) 
     = 35p^4 - 105p^5 + 105p^6 - 35p^7

c) 0.15 = 35p^4 - 105p^5 + 105p^6 - 35p^7
    p = 0.356 or

    p = 0.770