Let C's speed = x kmph,

B's speed = 3x kmph

& A's speed = 6x kmph

therefore Ratio of speeds of A, B, C = 6x :3x : x = 6:3:1

Ratio of times taken = 1/6 : 1/3 : 1 = 1 : 2 : 6

If C takes 6 min, A takes 1 min

If C takes 42 min, A takes (1/6 * 42) min = 7 min