x = current age
x+7 = age 7 years from now
x-3 = age 3 years ago
His age 7 years from now is (will be) twice his age 3 years ago:
Solve using basic algebra. I leave that to you.
Solving the problem informally, using logical reasoning and simple mental arithmetic, can be good brain exercise. Something like the following.
The difference between 7 years from now and 3 years ago is 10 years.
If the age 7 years from now is twice the age 3 years ago, and the difference is 10 years, then those two ages must be 20 and 10.
So 7 years from now he will be 20, and 3 years ago he was 10; either way, his current age is 13.
You of course should get that same answer using the algebraic solution method.
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