book Maths Puzzle Numerical Ability

How many three-digit counting numbers are even? How many five-digit counting numbers contain at least one 6?

I had the answer of 400 = 10 x 10 x 4, but I was told that this was

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book Other Question
simplify:

x^2(2x-5)^2+x(5-2x)^3

Possible answers:

a. x(2x-5)^2 (5-x)
b. x(2x-5)^2 (x-5)
c. -x(2x-5)^2 (5-x)
d. x(2x-5) (5-x) If A={x|x is an odd integer} and B={39, 41, 42, 44}, list the elements of A ⋂ B.