Question

How does an underestimate relate to rounding factors before estimating?

Answer 1

when both factors in a multiplication problem are rounded up to estimate the product, the estimate is an overestimate.

Answer 2

When you round up, you overestimate because now numbers are bigger than before rounding. In a similar way, when you round down, you underestimate. And, when you round down some factors and round up others, it is hard to tell whether the estimate is an overestimate or an underestimate.