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Aleksandr

a man is 4 times as old as his younger brother. After 5 years, he will be twice as old as his brother. Find his brother's present age. ​

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Answer:

My brother is 4 times as old as I am today. In 5 years time he will be twice as old as I will be. What will my age be in 10 years time?

Let's call your current age "y" and your brother's current age "b."

Today, your brother is four times your age. In other words, b=4y.

In five years, your brother's age will be twice, or two times, yours. In five years, your brother's age will be (b+5), and yours will be (y+5). Putting this information together, we get that b+5=2(y+5).

We see from the first equation that we can substitute b for 4y in the second equation. This gives us: 4y+5=2(y+5). This can be solved with some simple algebra:

4y+5=2y+10

2y=5

y=2.5

You are currently 2.5 years old. In 10 years, you will be 2.5+10=12.5 years old.

We can check this. If you're 2.5 today, your brother is 10 (2.5x4). In five years, you'll be 7.5, and he should be twice that, which is 15.

Indeed,10+5=15.

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