Galla

two ₹50s make a ₹100. that's the whole trick to multiplying in your head.

a customer pays in a pile of notes: drag one onto its twin to swap the pair for one note worth double, tap the total, and watch the shopkeeper write it down as 18 × 50 = 9 × 100

Two ₹50s are a ₹100, so 18 ₹50s are 9 ₹100s: half as many notes, each worth double, the same money. That is halve-and-double, the trick for multiplying in your head: 18 × 50 is 9 × 100, which is 900. After five customers the cash goes away and you do it in the rough book: halve one number, double the other, until one of them is 10, 100 or 1000.

Concepts: ratios, scaling, mental-math, money.

An interactive you play with one thumb, on Wundr.