gmat prep

[Riggz]

this is probably very easy, but I cant seem to figure it out once again.
please explain..tia!

[xilef]

Machine H harvested 40 in 8hrs, so H does 5 acres/hr

H and K did the remaining 60 acres in 5 hrs.

H did 25 acres in those 5 hrs, meaning that K did 35 in those 5 hrs or 7/hr.

so the answer is A.

[moneyman]

Please tell me what am I missing here.

I used the formula 1/H+1/K=1/T(total time)

According to the problem,

H harvests 5 acres per hour so 1/H=1/5

We dont know what 1/K is.

TOgether H and K harvest 60 acres in 5 hours so 12 per hour which means 1/T=1/12

1/5+1/K=1/12 which gives me 1/K=-7/60

Whats wrong in my approach..pls help..test date 10 days from now..trying to get my basics right!! thanks

[AleksandrM]

Treat this the following way:

Break down a table into three rows: r for rate, t for time, and w for work; and three columns: H for machine H, K + H for machine K and H working together, and T for total.

For H, you will have rate = 40/8 = 5 acres per hour. t = 8 hours and work = 40 acres.

For K + H, you will have rate = 5/1 + unknown. t = 5 and work = 60 acres

The total column will just have the total acres, 100. You could also use the total column for the two machines together, but you might get confused easily that way, and it is also a bit of a time issue.

Then set up an equation: w = rt or tr [doesn't matter]

60 = 5 (5/1 + r)

35 = 5r

r = 7 ------> 7/1, or 7 acres per hour.