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3 years ago

Four brothers and their sister have the average age 7. Four brothers have the average age 6. How old is the sister?

Mathematics

1 answer:

AnnyKZ [126]3 years ago

4 0

Answer:

11 years old

Step-by-step explanation:

If four brothers have the average age 6, then the sum of their ages is

$4\cdot 6=24$

years.

Let x years be the age of sister. If four brothers and their sister have the average age 7, then

$\dfrac{24+x}{5}=7.$

Multiply this equation by 5:

$24+x=35\Rightarrow x=35-24=11.$

The sister is 11 years old.

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N(N ∩ S ∩ K) = 10
n(ξ) = 250
n(S ∪ K) = 15 - 10 = 5
n(N ∪ S) = 20 - 10 = 10
n(N ∪ K) = 30 - 10 = 20
n(S) = 50 - 10 - 5 - 10 = 25
n(K) = 55 - 20 - 5 - 10 = 20
n(N) = 100 - 10 - 20 - 10 = 60

n(N ∪ S ∪ K) = 10 + 5 + 10 + 20 + 25 + 20 + 60 = 150

Therefore, n(N ∪ S ∪ K)' = 250 - 150 = 100

Therefore, 100 million people do not read any of the three papers.

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