Q.

Let X be the set of all five digit numbers formed using 1, 2, 2, 4, 4, 4, 0. For example 22440  is in X while 02244 and 22244 are not in X. Suppose that each element of X has an equal chance of being chosen. Let P be the conditional probability that an element chosen at random is a multiple of 20 given that it is a multiple of 5. Then the value of 76P – 53 is equal to _______

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answer is 9.

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Detailed Solution

event that 5 digit number is divisible by 5 last digit is 0 in all the numbers.
Number of ways = Coefficient of  x4  in  4!(x0+x1)(x0+x11!+x22!)(x0+x11!+x22!+x33!)=38
B event that 5 digit number is divisible by 20 last 2 digits are 20 (or) 40.
Number of ways = 
Coefficient of  x3  in  3!(x0+x1)(x0+x1)(x0+x1+x22!+x33!)  +
 Coefficient of  x3  in  3!(x0+x1)(x0+x1+x22!)(x0+x1+x22!)=13+18=31
 P=P(BA)=P(AB)P(A)=3138
 76P53=76(3138)53=6253=9

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