问问?没看懂咋算出来的答案?

问问?没看懂咋算出来的答案?Question #4:A cyclist bikes from town A to town B and back to town A in 3 hours.He bikes from A to B at a speed of 15 miles/hour,while his return speed is 10 miles/hour.What is the distance between the 2 towns?(a) 18 miles(b) 15 miles(c) 12 miles(d) 10 miles(e) 11 milesAnswer:The total time t = tAB + tBA = dAB/(15 miles/hour) + dAB/(10 miles/hour) = dAB/(6 miles/hour).dAB = dAB/(6 miles/hour) · 3 hours = 18 miles.(a) is the correct answer.--------------------------------------------------------------------------------