Cranes

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dlencz: Senior | Next Rank: 100 Posts; Posts: 42; Joined: Tue Dec 20, 2011 6:26 pm; Thanked: 2 times; Followed by:1 members

Cranes

by dlencz » Mon Apr 16, 2012 3:13 pm

By themselves, crane operators A, B, and C can load a particular ship with containers in 5 hours, 7 hours, and 8 hours, respectively. What is the ratio of the time that it takes crane operators B and C to load this ship, working together at their individual rates, to the time it takes crane operator A working alone at her respective rate?

(A) 12/13

(B) 58/75

(C) 56/75

(D) 56/99

(E) 56/113

OA is C

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Source: Beat The GMAT — Problem Solving | Mon Apr 16, 2012 3:13 pm

dlencz: Senior | Next Rank: 100 Posts; Posts: 42; Joined: Tue Dec 20, 2011 6:26 pm; Thanked: 2 times; Followed by:1 members

by dlencz » Mon Apr 16, 2012 3:16 pm

I got the correct answer, but my ratio was inverted.

When you combine B and C you get 15/56. The ratio between those two and A is then (15/56)/(1/5), correct? When I multiplied the top by 5/1 I got 75/56. I knew that the top number needed to be smaller so I went with C, but I'd like to understand why my ratio was inverted. What did I do incorrectly?

Thanks.