If N varies directly as M and N = 8 when M = 20 find M when N = 7
13
15
\(17\frac{1}{2}\)
\(18\frac{1}{2}\)
Correct answer is C
N ∝ M
∴N = kM where k is a constant
N = 8 when M = 20
∴ 8 = k x 20
\(k = \frac{8}{20} = \frac{2}{5}\\
∴ N = \frac{2}{5}M\\
If N = 7\\
∴ 7 = \frac{2}{5}M\\
M = \frac{35}{2} = 17\frac{1}{2}\)