Finding a Missing Value

Often the letter you want is not the one on its own. Take this question: a car travels 120 km at 40 km per hour, and you want to know how long the trip takes. Substitute $d = 120$ and $s = 40$ into the distance formula $d = st$:

$$120 = 40t$$

The result is a one-step equation for $t$. Divide both sides by 40 to get $t = 3$, so the trip takes 3 hours. The formula itself did not change. You substituted the values you knew and solved for the letter that was left, the same way as with any equation.

One formula can answer three different questions. With $d = st$ you can find the distance, the speed or the time, depending on which letter is unknown. You solve all three the same way: substitute the values you know, then solve for the letter that is left.