Direct and Inverse Proportion - JSS3 Mathematics Lesson Note

Direct Proportion: Two quantities are in direct proportion if increasing one increases the other at a constant rate. Mathematically, 𝑦∝𝑥 or 𝑦=𝑘𝑥, where 𝑘 is a constant.

Example 1: If 5 pens cost $10, how much do 8 pens cost?

Find the cost per pen: 

10/5 = 2 dollars per pen.

Cost of 8 pens: 

8×2=16 dollars.

Inverse Proportion: Two quantities are in inverse proportion if increasing one decreases the other at a constant rate. Mathematically, 𝑦∝1𝑥  or 𝑦=𝑘𝑥.

Example 2: If 5 workers can complete a task in 10 days, how many days will it take for 10 workers to complete the same task?

Use inverse proportion: 

5×10 =10×𝑑

Solve for 

𝑑: 𝑑=50/10 =5 days.