Mahesh can walk from his house to the railway station in 30 minutes, and a cycle rickshaw covers the same road in 12 minutes. One morning he sets out walking, and after 10 minutes he stops a rickshaw and rides the rest of the way. He reaches the station in how many minutes altogether?

In 10 of the 30 minutes he covers one-third of the road on foot, leaving two-thirds for the rickshaw, which needs $2/3\times 12=8$ minutes. Total 10 + 8 = 18 minutes. (Adding a full rickshaw ride to the walk gives 22 minutes and forgets that he had already covered part of the road; charging the rickshaw with half the road gives 10 + 6 = 16 or, with half the rickshaw's time, 10 + 10 = 20. The plain average of the two whole-journey times, 21 minutes, has no bearing on a journey split in this way.)