Q: Mary takes 15 minutes to eat a plate of french fries. Ann takes 30 minutes to eat a plate of french fries. If Mary and Ann decide to eat fries, from the plate, each for a minute alternatively starting with Mary, how long will it take them to finish the plate, if each eats the french fries at her respective rate?

(Target Audience: GMAT)

A) 8 mins
B) 10 mins
C) 18 mins
D) 20 mins
E) 45 mins

Explanation: Generic Way

Mary eat 1 P in 15 min
Therefore, Mary’s rate = 1P/15min =
Or, M’s rate = 1/15
Ann eat 1 P in 30 min
Therefore, Ann’s rate = 1P/30min =
Or, A’s rate = 1/30
First minute: Mary eats 1/15 plate
Next minute: Ann eats 1/30 plate
So after two minutes, they have eaten 1/15+1/30=3/30=1/10 plate.

How long will it take them to eat 1 plate?
1/10 Plate in 2 min,  or 1/20 plate in 1 min.
Therefore, 20 min for 1 plate

OPTION: D

Explanation: HolaMaven Way

Let the total fries in the plate are 30
Mary eats 2 fries per minutes
Ann eats 1 fries per minutes
so in 2 minutes they eat 3 fries.
To eat 30 fries they need 20 minutes

OPTION: D