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

100 numbers

Step-by-step explanation:

There are 100 numbers with 5 as the hundreds digit. (500-599)

There are 100 numbers with 6 as the hundreds digit. (600-699)

There are 50 even numbers with 5 as the hundreds digit.

(500, 502, 504, ..., 598)

There are 50 even numbers with 6 as the hundreds digit.

(600, 602, 604, ..., 698)

50+50=100 numbers