Question

Suppose that r varies directly with s and inversely with t, and r = 2 when s = 3 and t = 12. What is the value of r when s = 5 and t = 4?

2/5
32/5
10
20

Answer 1

2 * (3/2) = 3 s
2 * 6 = 12 t

3 (5/3) = 5 so 2(5/3) = 10/3 [s] <--- Directly
12 (1/3) = 4 so (10/3)(3/1) = 10[s] <--- Inverse

Answer is 10.