Question

Sarah is twice as old as her youngest brother if the difference between their ages is 15 years how old is her youngest brother

Answer 1

He is 15 years old.
You would set up a system of equations
S = Sarah
Y = Youngest brother

S = 2Y
S-Y= 15

Substitute 2Y for S
So 2Y-Y= 15
Y= 15
and that's your answer :-)