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Hey, I understand that you translate from your native language, which is not English.
English is VERY SPECIAL language, and has many features that other languages do not have.
And in opposite, it HAS NO some forms, that other languages have.
For example, this combination of words "a boy is 3 times younger than his uncle" HAS NO SENSE in English.
Instead of it, English-speaking people say "the uncle is three times as old as the boy" (!)
Then it makes sense (!) (!)
So, based on it, the answer to your problem is 16 years for the boy and 3*16 = 48 years for the uncle.
How to get it ? --- Very easy . . .
Let x be the age of the boy; then the age of the uncle is 3x, and
x + 3x = 64
4x = 64
x = 64/4 = 16
and 3x = 48.
Learn English !
Learn it from all possible sources --- from my post, for example (!)
Happy learning (!)
From which language do you translate ?