JAMB 2011 · UTME · Q17

$T$ varies inversely as the cube of $R$ . When $R = 3$ , $T = \frac{2}{81}$ . Find $T$ when $R = 2$ .

Worked solution (try it first)

$T = \dfrac{k}{R^3}$ , so $k = TR^3$ .
With $R = 3$ : $k = \frac{2}{81} \times 27 = \frac23$ .
When $R = 2$ : $T = \dfrac{2/3}{8} = \dfrac{2}{24}$ , which is $\dfrac{1}{12}$ , option B.