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by prachi18oct » Sun Jul 12, 2015 10:23 am
Richard has to paint a mural with seven horizontal stripes. He only has enough paint for four red stripes, four blue stripes, four white stripes, four black stripes, and four yellow stripes. If his patron wants at most two different colors in the mural, how many different ways can he paint the wall?
A 120
B 350
C 700
D 2,520
E 5,040

Richard can only have exactly 2 paints as if he chooses one paint he wont have enough of it to paint 7 stripes.
so to choose 2 out of 5 => combinations => 5C2 = 10
As the seven strips can be painted in 3-4 combination of two paints
so arrangements possible = 7!/ 4! 3! = 35
SO total ways = 35 * 10 = 350 but OA given is 700

Please suggest.
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Source: — Problem Solving |

by Jim@StratusPrep » Sun Jul 12, 2015 11:53 am
prachi18oct wrote:Richard has to paint a mural with seven horizontal stripes. He only has enough paint for four red stripes, four blue stripes, four white stripes, four black stripes, and four yellow stripes. If his patron wants at most two different colors in the mural, how many different ways can he paint the wall?
A 120
B 350
C 700
D 2,520
E 5,040

Richard can only have exactly 2 paints as if he chooses one paint he wont have enough of it to paint 7 stripes.
so to choose 2 out of 5 => combinations => 5C2 = 10
As the seven strips can be painted in 3-4 combination of two paints
so arrangements possible = 7!/ 4! 3! = 35
SO total ways = 35 * 10 = 350 but OA given is 700

Please suggest.

You need to multiply 35 by 2. Let's look at Black and White. You either have:

4B and 3W:

BBBBWWW

or

4W and 3B:

WWWWBBB.

EACH ONE OF THESE has 35 different combinations. 35 + 35 = 70

70 x 10 = 700
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