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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