Simultaneous rates? easy help

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coolgirl26: Senior | Next Rank: 100 Posts; Posts: 71; Joined: Tue Mar 03, 2009 1:24 pm

by coolgirl26 » Mon Apr 13, 2009 11:01 am

How to do simultaneous rates?

i thought you add just them and solve

20 minutes= 1/3 of an hour
30 minutes = 1/2 of an hour

1/2+1/3= 5/6 x?

d=r*t
1= 5/6t

but my answer is wrong!

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Source: Beat The GMAT — Problem Solving | Mon Apr 13, 2009 11:01 am

mike22629: Master | Next Rank: 500 Posts; Posts: 322; Joined: Fri Mar 27, 2009 3:56 pm; Thanked: 24 times; GMAT Score:710

by mike22629 » Mon Apr 13, 2009 11:05 am

Use (1/a) + (1/b) = (1/t) formula

(1/20) + (1/30) = 5/60

60/5 = 12

Answer 12 minutes

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bronzie35: Junior | Next Rank: 30 Posts; Posts: 18; Joined: Fri Apr 03, 2009 3:33 pm

by bronzie35 » Mon Apr 13, 2009 11:32 am

How do you copy the practice test problem and attach it here? I've tried over and over to copy but I can not figure out how to do it.

Thanks.

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truplayer256: Master | Next Rank: 500 Posts; Posts: 392; Joined: Thu Jan 15, 2009 12:52 pm; Location: New Jersey; Thanked: 76 times

by truplayer256 » Mon Apr 13, 2009 12:26 pm

Aside from Mike22629's formula to solve this problem, this problem can also be solved by doing the following:

Let the depth of the pool equal x.

The smaller hose can fill the pool at x/30 per min and the bigger hose can fill the pool at x/20 per min. From this, we can conclude that both hoses can fill the pool at x/30+x/20= x/12 per min simultaneously. Now, we need to know how many minutes it'll take the fill the pool, which is of depth x. Since both hoses fill the pool at x/12 per min, we can do (x)/(x/12) to find out how many mins it'll take them to fill the whole pool. The answer comes out to be 12.

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