Question

what can be defined as "numbers that are close to the given numbers that make estimation or mental calculation easier"?

Answer 1

approximations?

Answer 2

For example, the car lot has 31 cars that each cost $19,000, then the total cost of the cars is approximately 30 x $20,000 = $600,000.

This is quite close in this case as we 'rounded down' 31 -> 30 and 'rounded up' 19 -> 20. These approximations helped cancel each other out. The exact answer is
31 x $19,000 = $589,000
a difference of around 2%