19. If 17 labourers can dig a ditch 26 metres long in 18 days, working 8 hours a day, how many more labourers should be engaged to dig a similar ditch 39 metres long in 6 days, each labourer working 9 hours a day ?

20. A contract is to be completed in 56 days and 104 men were set to work, each working 8 hours a day. After 30 days, of the work is completed. How many additional men may be employed, so that the work may be compeleted in time, each man now working 9 hours a day ?

21. A garrison had provisions for a certain number of days. After 10 days (1/5)th of the men desert and it is found that the provisions will now last just as long as before. How long was that :