okay so lets call Leah "L" and her cousin "C". We know that L+C=36 ... we also know that Leah is twice her cousins age. Therefore, L=2 times C, or L=2C. This is because Leah's age is equivalent to twice as much as her cousin's.
Now that you know that L=2C, you can plug this back into the equation. This should make it so that's there's only one variable now!
L+C=36
(2C)+C=36 ... here we subbed in L=2C
3C=36 ... we add up the C's
C=12 ... we isolate for C by dividing both sides by 3
So her cousin's age is 12 years old. Leah's age is twice that. Thus, she's 24. If you add the two up: 12+24, you indeed get 36. Hope that helps :))
