4 men and 6 boys can finish a piece of work in 5 days while 3 men and 4 boys can finish it in

7 days. Find the time taken by 1 man alone or that by 1 boy alone.

4/x + 6/y = 1/5

3/x + 4/y = 1/7

let 1/x be u and 1/y be v

then, 

4u + 6v = 1/5               20 u+30v =1 ............(1)

3u + 4v = 1/7                21u +28v =1.............(2)

then by elimination method,

21(20u+31v = 1) .

20(21 u+28v =1)

420u -420u +630v - 560 v  =21-20

70 v = 1        v = 1/70 and y =70 days

x = 35 days

1 man can do work in 35days

1 boy can do work in 70 days