You can flip which number is the percentage — because a%·b = (a·b)/100 = b%·a — turning a hard percentage into an easy one.

• 8% of 25 looks awkward → swap to 25% of 8 = a quarter of 8 = 2.
• 4% of 75 → 75% of 4 = 3.
• 18% of 50 → 50% of 18 = 9.

Both sides are literally the same product over 100, so the answer is identical — you just pick the orientation your head finds easier (often one side becomes "a half / quarter / tenth of a small number").

Round each number to something easy, compute, and track whether you rounded up or down so you know which way the true answer leans.

• 312 + 488 → 300 + 500 = ~800 (actual 800).
• 47 × 19 → 47 × 20 = 940, but you rounded 19 up by 1 (≈ one 47 too many), so the true value is a bit under 940 (actual 893).

Rounding both numbers the same direction inflates or deflates the estimate predictably; rounding in opposite directions tends to cancel and gives a tighter estimate.