Up Your Score SAT, 2018-2019 Edition The Underground Guide to Outsmarting The Test

Another arithmetic cheat: Check the last digits. If you’re multiplying two
large numbers, you can figure out what the last digit of the answer is without
multiplying them completely. Multiply the end numbers together and take the
last digit of this product. Let’s try this out.

23  ×   257 =

A)  5,911

B)  5,312

C)  4,517

D)  5,118

Without multiplying it out, let’s do a last-digit check: 3 × 7 (the last two
numbers) = 21; last digit = 1. Only choice A ends in 1, so the answer is A.

MENTAL MATH TRICK 4: KNOW YOUR SQUARES

We are going to talk about exponents and square roots at a later point in this
chapter, but you should memorize the basic squares. We assume you know the
squares of 1 through 10, but here’s 11 through 20, plus 25. These will not only
help you speed through problems that involve squares, but will also help you
decode sophisticated-seeming problems.

Number Square

11 121

12 144

13 169

14 196

15 225

16 256

17 289

18 324