Edexcel IGCSE Math A 2.5 Proportion Question Bank

Practise forming direct and inverse proportion formulae, finding constants, and using powers or roots to calculate missing values.

Exam points

set up y=kx^n or y=k/x^n from wording, then find k from a given pair of values
calculate a missing variable by substituting into a proportion formula with squares, cubes or roots

[Maximum number: 4]

18 y is inversely proportional to $x^{3}$

y=4 \text { when } x=3

Work out the value of x when y=864

Since y is inversely proportional to $x^3$ ,

y=\frac{k}{x^3}

4=\frac{k}{3^3} \quad\Rightarrow\quad k=4\times3^3=108

864=\frac{108}{x^3} \quad\Rightarrow\quad x^3=\frac{108}{864}=\frac18

Final answer:

x=0.5

M1 for a correct inverse-proportion formula with a constant k.
M1 for substituting x=3 and y=4.
M1 for a method to find $x^3$ .
A1 for 0.5 oe.

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