Estimating in maths requires a strong knowledge of rounding as that is often the best way to ensure you don’t make a mistake in your estimation. For example, if asked to estimate the answer for 0.7 x 0.8, by rounding the decimals and then multiplying (0.7 and 0.8 both round up to 1, and 1 x 1 = 1) you are better able to check the accuracy of the answer (5.6 is much larger than 1, so the multiplication could be recalculated to achieve 0.56, which is much closer to 1).