Math 1 · Unit 8 · Lesson 1 · part 4 of 5
Solve the formula for any letter
The formula I = Prt has four letters.
Solve the formula for any letter
The formula I = Prt has four letters. Know any three and you can find the fourth, using the same move as Unit 1, Lesson 4: undo multiplying by dividing, and do it to both sides.
Solve for r
I = Prt. The r is multiplied by P and by t. Divide both sides by Pt:
r = IPt
Solve for t
Divide both sides by Pr:
t = IPr
Solve for P
Divide both sides by rt:
P = Irt
Worked: what was the rate?$600 earned $54 of simple interest in 3 years. r = IPt = 54600 × 3 = 541800 = 0.03. As a percent, 0.03 is 3%. Check it forward: 600 × 0.03 × 3 = 54. It works.
Same moves, new letters. Solving a formula for a letter is solving an equation. Whatever is multiplied onto the letter you want, divide both sides by it, and write the result as a stacked fraction.