Question 498262
Let Kelly's sister earns = x

Kelly earns = (x*40%)+x
Kelly earns = x*(40/100)+x
Kelly earns = x*(2/5)+x
Kelly earns = (2x/5)+x

Together both earns = 720
x+(2x/5)+x=720
2x+(2x/5)=720

Multiply by 5 both sides
10x+2x=3600
12x=3600
Divide by 12 both sides
12x/12=3600/12
x=300
Kelly's sister earns = 300$

Kelly earns = x*(2/5)+ x
Put the value of x in above equation
Kelly earns = 300*(2/5)+300
Kelly earns = 60*(2)+300
Kelly earns = 120+300
Kelly earns = 420$