The double-discount trap
"30% off + extra 10% off" is not 40% off. The second discount applies to the already-reduced price: a $100 item becomes $70, then $63 — an effective 37%. Retailers know most shoppers add the numbers; now you don't.
Quick mental math
10% = move the decimal one place. 25% = quarter of the price. For anything else, this page is faster than your phone's calculator app.