Hi,

Let x represent sister's age   ⇒   is Frank's age

Question States*** in 3 years...

 (x+8) + 3 = 2(x+3)

   x + 11 = 2x + 6

        5 = x, sister's age.  frank is 13

CHECKING our answer*** In 3 years, Frank will be twice sister's age.

  16 = 2•8

Wish You the Best in your Studies. 

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