Twelve labourers of equal skill were engaged to finish a job in 20 days. At the end of the fifth day two of them left and were not replaced. By how many days does the job overrun its 20-day schedule?

The job is $12\times 20=240$ labourer-days. Five days with twelve men account for 60, leaving 180 for the ten who stay: 180 ÷ 10 = 18 days more, so the job runs 5 + 18 = 23 days in all and overruns by 3 days. 18 days is the closing stretch, not the overrun. 23 days is the new total, again not the overrun. 4 days rescales the whole 20-day schedule by 12/10 to get 24 days, which wrongly applies the reduced gang to the five days already worked at full strength.