GRE Exam Numerical Ability Time and Work

grass in lawn grows equally thick in a uniform rate.it takes 24days for 70 cows and 60days for 30 cows to eat the whole grass.how many cows are needed to eat the grass in 96days?

Read Solution (Total 1)

Let a be the initial quantity (in relevant units) of grass and b be the amount of grass grown per day.
Then, 60 x 30 = 1800 = a + 60b and 24*70 = 1680 = a + 24b. Solving these equations, we get
36b = 120
b = 10/3, a = 1600
x*96 = 1600+96(10/3)
x = 1920/96 = 20.
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GRE Other Question

if 20 men or 24 women or 40 boys can do a job in 12 days working for 8hours a day.how many men working with 6 women and 2 boys take to do a job four times as big working 5 hours a day for 12 days? if A and B work together,they will complete a job in 7.5days.however,if A works alone and completes half the work and then B takes over and completes the remaining half alone,they will take 20days.how long will B alone take to do the work if A is more efficient than B ?