Question 1200074
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<pre>

The equation in your post

    x + (3x+3) = 23

is correct.


Continue to solve it

    x + 3x = 23 - 3

     4x    =   20

      x    =   20/4 = 5.


The boy is 5 years old.

The sister is  3*5+3 = 18 years old.


<U>CHECK</U>.  5 + 18 = 23  the sum of their ages.    ! correct !
</pre>

Solved, with explanations.


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