If 3men with 4 boys earn rs.2700 in 4 days and 2 man with 3 boys earn rs.1900 in same time , in what time wil

I'm guessing all men make one amount of money per hour & all boys make a second amount. M = amount men make per day, B = amount boys make per day

4 days * (3M + 4B) = 2700
so 12M + 16B = 2700

4 days * (2M + 3B) = 1900
so 8M + 12B = 1900

So now you have to solve the two simultaneous equations. Multiply the second one by 1.5, then each will have 12M & you can subtract one from the other to eliminate M.

1.5*(8M + 12B) = 1.5*1900
12M + 18B = 2850

Now subtract the second equation from the first

12M + 16B = 2700
-
12M + 18B = 2850
=
12M - 12M + 16B - 18B = 2700 - 2850

-2B = -150

B = (-150)/(-2) = 75, so a boy makes rs.75 per day

Plug that into any of the equations to find what a man makes

12M + 16B = 2700

12M + 16(75) = 2700

12M + 1200 = 2700

12M = 1500

M = 1500/12 = 125, a man makes rs.125 per day

How long will it take for 4 men & 4 boys to earn rs.8000?

We will say it is D number of days

D days * (4 * 125 + 4 * 75) = 8000

D * (500 + 300) = 8000

800D = 8000

D = 10 days