5 men and 8 women can finish a harvesting job in 6 days, and 4 men and 12 women can also finish it in 6 days. In how many days can 10 men finish the same job?

The two crews take the same time, so their daily outputs are equal: $5m+8w=4m+12w$, giving $m=4w$ — one man matches four women. The first crew is then $5(4w)+8w=28w$, and the job is $28\times 6=168$ woman-days. Ten men are $40w$, so 168 ÷ 40 = 4.2 days. 16.8 days reads "10 men" as ten women. 8.4 days values a man at two women, 3.36 days at five; only $m=4w$ satisfies the two given crews.