Leah's age is twice her cow old are leah and her cousin? usin's age. if they add their ages together they get 36. how old are leah and her cousin

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 :))