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# In a certain high school, 80 percent of the seniors are taki

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In a certain high school, 80 percent of the seniors are taki [#permalink]  26 Jun 2020, 09:39
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Question Stats:

68% (01:07) correct 31% (01:22) wrong based on 22 sessions
In a certain high school, 80 percent of the seniors are taking calculus, and 60 percent of the seniors who are taking calculus are also taking physics. If 10 percent of the seniors are taking neither calculus nor physics, what percent of the seniors are taking physics?

(A) 40%
(B) 42%
(C) 48%
(D) 58%
(E) 80%
[Reveal] Spoiler: OA

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Founder
Joined: 18 Apr 2015
Posts: 13367
Followers: 290

Kudos [?]: 3398 [0], given: 12223

Re: In a certain high school, 80 percent of the seniors are taki [#permalink]  26 Jun 2020, 09:40
Expert's post
Post A Detailed Correct Solution For The Above Questions And Get A Kudos.
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Re: In a certain high school, 80 percent of the seniors are taki [#permalink]  26 Jun 2020, 09:51
Expert's post
Carcass wrote:
In a certain high school, 80 percent of the seniors are taking calculus, and 60 percent of the seniors who are taking calculus are also taking physics. If 10 percent of the seniors are taking neither calculus nor physics, what percent of the seniors are taking physics?

(A) 40%
(B) 42%
(C) 48%
(D) 58%
(E) 80%

One approach is to use the Double Matrix Method. This technique can be used for questions featuring a population in which each member has two characteristics associated with it (aka overlapping sets questions).
Here, we have a population of seniors, and the two characteristics are:
- taking calculus or not taking calculus
- taking physics or not taking physics

Since we are asked to find a certain PERCENT, let's say that there are 100 seniors in the school.

Our initial diagram looks like this:

80 percent of the seniors are taking calculus
So 80 seniors are taking calculus, and the remaining 20 seniors are not taking calculus. We get:

60 percent of the seniors who are taking calculus are also taking physics.
We know that 80 seniors are taking calculus
60 percent of 80 = 48
So, 48 seniors are taking calculus AND physics. We get:

If 10 percent of the seniors are taking neither calculus nor physics, what percent of the seniors are taking physics?
10% of 100 = 10
So 10 seniors are taking NEITHER calculus NOT physics. We get:

What percent of the seniors are taking physics?
When we complete the rest of the diagram we get:

We can now see that 58 of the 100 seniors are taking physics.
In other words, 58% of the seniors are taking physics

Aside: We can also use Venn diagrams and formulae to solve overlapping sets questions. However, as difficulty levels increase, it becomes harder to apply those other approaches, whereas the Double Matrix Method works every time.

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Re: In a certain high school, 80 percent of the seniors are taki   [#permalink] 26 Jun 2020, 09:51
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