Question 1023594
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The sum of the ages of two brothers is 31 years. They differ in age by 3 years. How is old is each of them?
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x + y = 31,   (1)
x - y =  3.   (2)

Every time when you see two equations like these, add them. You will get

2x = 31 + 3,
2x = 34,

x = {{{34/2}}} = 17.

Thus the older brother is 17 years old.
The younger brother's age is 17-3 = 14 years.
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