WAEC - Mathematics (2001 - No. 5)
If N varies directly as M and N = 8 when M = 20 find M when N = 7
13
15
\(17\frac{1}{2}\)
\(18\frac{1}{2}\)
Explanation
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 = \(\frac{2 M}{5}\)
If N = 7
∴ 7 = \(\frac{2 M}{5}\)
M = \(\frac{35}{2}\) = 17\(\frac{1}{2}\)
∴N = kM where k is a constant
N = 8 when M = 20
∴ 8 = k x 20
\(k = \frac{8}{20}\) = \(\frac{2}{5}\)
∴ N = \(\frac{2 M}{5}\)
If N = 7
∴ 7 = \(\frac{2 M}{5}\)
M = \(\frac{35}{2}\) = 17\(\frac{1}{2}\)
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