a. What is the probability that A and B call the same number?
b. What is the probability that A and B call a specific number?
the probability that A calls a specific number:1/200
the probability that B calls the same specific number: 1/200
1/40000 is the probability of both A and B calling the same, prespecified.
This might be confusing, but try to look at it this way, and assume the numbers are simply 1,2,3,4...200
A could call any of these numbers, so there's a 1/200 chance of A calling a specific number, say, #153
B could call any of these numbers, so there's a 1/200 chance of B calling a specific number, say, #153
1/40000 is the chance of A and B both calling #153.
but, the odds of A and B calling the same number (any number) is still 1/200.
I can try again if this makes no sense.
No matter what number A calls, there is a 1/200 chance that B will match it. Hence, the prob is 1/200.