Normal method:S'pose one soldier eats 1kg per day, so 600 soldiers would eat 600*30*1 = 18000 kgs in 30 days.
That is 600 kgs per 600 soldiers per day. 200 more soldiers join after 10 days, so ration consumed in these 10 days = 600*10 = 6000 kgs.
Remaining ration = 18000 - 6000 = 12000kgs.
If no one joined, this 12000 kgs would have been consumed by 600 soldiers in 20 days (1kg/soldier).
But, the total no. of soldiers now = 800.
So, 12000 kgs of ration will now be consumed by 800 soldiers in 12000/800 = 15 days.
So, the ratio will now last for 15 days instead of 20.
Faster method:Let the rate of consumption for each soldier be 1 kg/day.
Total ration = 600*30*1 = 18000 kgs. This is a Work-Rate equation. 600 soldiers with rate of consumption of 1 kg/day take 30 days to consume 18000 kgs.
In 10, 600 soldiers at a rate of consumption 1 kg/day would consume = 600*10*1 = 6000 kgs of ration.
So, 800 soldiers with the same rate of consumption of 1 kg/day will take \(x\) days to consume the remaining 12000 kgs of ration.
800*1*\(x\) = 12000.
\(x\) = 15 days.