Finding Percents of Numbers by Starting from Ten Percent
About this lesson
Students find 10% of a number by dividing by 10. With a partner, they use that answer to work out other percents, such as 5%, 20% and 15%, and then explain their thinking. It takes about 12 minutes and needs only the board, scrap paper and pencils.
- Grade
- 6
- Subject
- Math
- Time
- 12 min
- Prep
- 2 min
Objective
Students will find 5%, 10%, 15%, 20% and 25% of whole numbers using mental strategies that start from 10%.
Prep
2 min: write the numbers 60, 140, 250, 36, 480 and 90 on the board
Materials
- whiteboard and marker
- scrap paper
- pencils
Steps
- Write 80 on the board and ask, "What is 10% of 80?" Show that 10% means dividing by 10, which gives 8. Then ask the class how 8 can help them find 5%, 20% and 15% of 80 (half it, double it, add 10% and 5%).
- Pair students and have them draw a table on scrap paper with the columns 10%, 5%, 20%, 15% and 25%, with one row for each number on the board.
- For each row, Partner A finds 10% and Partner B uses that answer to fill in the other percents out loud. Partners switch roles on every new row.
- After about 6 minutes, call out a number and a percent, such as "15% of 140." Pairs show their answer on paper, and one pair explains how they built it from 10%.
- Finish with a challenge for the whole class. Ask, "How could you find 35% of 60?" Take two or three different strategies.
Quick Check
Each student writes the answer to 15% of 40 on scrap paper, with one sentence explaining how they got it (10% is 4, 5% is 2, so 15% is 6), and hands it in on the way out.
Differentiation
Make it easier: Use only 10% and 20% with multiples of 10, such as 50, 70 and 300.
Make it harder: Add 12.5% and 1% columns, or work backwards with questions like "10% of a number is 7; what is the number and what is 35% of it?"
Good For
- Before lunch
- End of day
- Supply teacher
- Review
