Kesang's recipe uses 10 spoons of sugar for 6 glasses of lemonade. Her father expects more guests and asks her to make an 18-glass batch instead. How much sugar keeps it equally sweet?

Nitin built the longer wall (60 ft) using 3 bags of cement. Hari built the shorter wall (40 ft) using 2 bags. Nitin worries Hari's wall is weaker, since Hari used less cement overall. Is he right to worry?

A car travels 90 km in 150 minutes. At the same speed, how far does it travel in 4 hours?

Around 499 CE, the Indian mathematician Āryabhata described exactly this kind of problem using three known numbers — the pramāṇa (measure), the phala (fruit/result), and the ichchhā (the new amount asked about) — to find a fourth, the ichchhāphala (the yield):

Multiply the phala by the ichchhā, and divide by the pramāṇa:

In the car example: pramāṇa = 150, phala = 90, ichchhā = 240, so ichchhāphala $=\frac{90\times 240}{150}=144$ km — exactly the cross-multiplication answer, 1500 years earlier.

Manjunath's regular filter coffee mixes 15 mL of decoction with 35 mL of milk — ratio $15:35$, simplest form $3:7$. A "stronger" cup uses 20 mL decoction with 30 mL milk.