Jeanine is twice as old as her brother marc. if the sum of their ages is 24. how old is jeanine?

Let Marc's age be x. If Jeanine is twice as old as Marc, then her age will be 2x. If their ages added together are 24, then you can create the following equation:

2x + x = 24
3x = 24
x = 8

Remember that x is Marc's age. So if Marc is 8, and Jeanine is twice as old, she will be 16. 

Subtracting is the opposite of adding

He was paid 23 dollars for each lawn