Seventh Grade · Math · week 6

Simple interest

Compute simple interest using I = P x r x t, and find the total amount in an account or loan.

Simple interest is money earned (or owed) just for the passage of time on a starting amount, called the principal (P). The formula is I = P x r x t, where r is the yearly interest rate written as a decimal, and t is time in years.

The TOTAL in an account after interest is simply the principal plus the interest: total = P + I. This is the same percent-of-a-number skill from the last two weeks, now spread out over time.

📋 For the teacher
Teach it (10–15 min):
  • Say the formula out loud as "principal times rate times time" before touching any numbers.
  • Always convert the percent rate to a decimal FIRST (5% becomes 0.05) -- this is the most common slip.
  • Keep interest (I) and total (principal + interest) clearly separate; a problem may ask for either one.
  • Try working the formula backwards (like solving for t) once fluency with the forward direction feels solid.
Talk about it:
  • What do P, r, and t each stand for in the simple interest formula?
  • Why must the rate be written as a decimal before multiplying?
  • What is the difference between the "interest" and the "total amount"?

Mastery looks like: They correctly apply I = P x r x t to find interest or total amount, and can solve for a missing piece like t.

Common stumbles: Forgetting to convert the percent to a decimal; confusing the interest amount with the final total.

See it visually

Interactive, spoken-aloud tools that show this idea in motion.

Counting & Ten-FramesCount objects in ten-frames and base-ten blocks, one tap at a time.try →
Homework for this lessonType it, write it with a stylus, or print it for pencil & paper.open →
Records attendance + 15 min of Math for this learner.
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