Explanation

This tests combining two ratios through their common term. From a:b = 3:5 = 21:35 and b:c = 7:8 = 35:40, the combined ratio is a:b:c = 21:35:40. Then 2a:3b:7c = 42:105:280, and dividing by the common factor 7 gives 6:15:40. The trap is option (a) 42:105:320, which comes from miscomputing 7c; the careful step is fixing b at its common value 35 before scaling. Hence option (c).