What number are easy to compute mentally?

Answer 1

The answer depends on your learning and expertise. For example, I had to learn my times table up to 30 times 10 and so I can do those multiplications easily. Most students nowadays do not learn tables for large numbers.

Also, I completed my basic studies before calculators and computers were in widespread use and so relied on manual calculations and that gave me lots of practice in arithmetic. So computations that I might consider trivially simple may look difficult to others.