Temperature

The formula $F = \frac{9}{5}C+32$ converts a Celsius temperature to Fahrenheit. Water boils at 212 °F. To find that temperature in Celsius, substitute $F = 212$:

$$212 = \frac{9}{5}C+32$$

Subtract 32 from both sides to get $180 = \frac{9}{5}C$. Then multiply both sides by $\frac{5}{9}$ to undo the fraction coefficient, which gives $C = 100$. Water boils at 100 °C, so the answer is right.

The equation $212 = \frac{9}{5}C+32$ is a two-step equation with a fraction coefficient, and the letter is on the right side. Look at what is done to $C$: it is multiplied by $\frac{9}{5}$, and then 32 is added. Undo those two steps in reverse order: subtract 32 first, then multiply by $\frac{5}{9}$.