How many 3-digit even numbers are that begin with either 5 or 6?

100 3 digit even number that begin with either 5 or 6

Explanation:
Case 1:
A 3 digit that begin with 5 should be 500-599 so there are 100 numbers
But we are only finding even number
So 100/2 = 50 3-digit even number that begin with 5.

Case 2:
A 3 digit that begin 6 should have
600-699 and there are also 100 number including even and odd in total. Since we are only finding even number we get:
100/2 = 50 3-digit even number that begin with 6.

Plus both case together we get 50 3-digit even number that begin with 5 + 50 3-digit even number that begin with 6= 50 + 50 = 100