Lets represent Ana and Christion using the Letters A and C a+c=500 c=a+150 now substitute c for a+150 back into the first equation a+a+150=500 a+a=350 2a=350 Divide by two a=175 Now that we know annie has 175 all we do is subtract that from the total (500) C=325 Sure enough, 175 is 150 less than 325 Hope that helped, send me a message if you need clearing up :D