Answer :

x-y=21 ⇒ x=y+21

x+13=2y

Substitute y+21 for x into the equation x+13=2y

So y+21+13=2y

Simplify 34=y

So Danielle is 34 years old. Plug y back in to the original equation

x-34=21

x=55

Tony is 55 years old


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