Because the average speed = total distance ÷ total time, the distance from bottom to top can be regarded as 1, the uphill time is 1 ÷ 6 = 1/6, and the downhill time is1÷12 =.

Formula: (1+1) ÷ (1/6+1/2) = 2 ÷ 3/12 = 2x12.