A two-step equation like 2x + 3 = 11 undoes in reverse order: subtract the 3 first, then divide by 2. An equation like 3(x + 2) = 15 can be divided first or multiplied out first. Rewriting an expression in a different form, like a + 0.5a = 1.5a, shows what a puzzle is really saying. With fractions and decimals, an estimate made first tells you whether the exact answer makes sense.