Direct and Inverse Proortion

Secondary School Maths

Example 1: Directly Proportional

W is directly proportional to H. When W = 56, H = 8. Find W when H = 4.

W ∞ H

Therefore: W = kH (k is the constant of proportionality introduced)

56 = k (8)

k = 568

k = 7

Therefore: when H = 4,

W = 7H

W = 7 x 4

W = 28



Example 2: Inversely Proportional

B is inversely proportional to T. When B = 80, T = 40. Find B when T = 10

B ∞ 1T

Therefore: B = kT

80 = k40

k = 80 x 40

k = 3200

Therefore: When T = 10

B = 3200T

B = 320010

B = 320



Try the questions below from Edexcel GCSE Exam Papers, UK: