Isobel and marnie volunteer at a daycare center. isobel works 3 hours for every 2 hours marnie works. if the 2 girls together work 20 hours one month, how many hours does isobel work?

Isobel =I  
Marnie =M
I(X) + M(X) = 20
3x + 2x =20
5x =20
divide by 5 both sides
5x/5 = 20/5
x = 4
then multiply I hours by 4  and M hours by 4

3(4) + 2(4) =20
12+ 8 = 20

Isobel worked a total of 12 hours and Marnie worked a total of 8.  
Hope this helps