Explain how you know whether an estimate of a product is an overstimate or an understimate

If the numbers contributing to your estimate were rounded up, the estimate is an overestimate. If they were rounded down, it is an underestimate. 

If one was rounded one way, and the other was rounded the other way, the estimate could be either over or under. In that case, it may be helpful to examine the percentage that each of the original numbers was changed in order to get to theestimated value. As a first approximation (not entirely accurate), you can add the percentage errors to determine whether the estimate is too high or too low.