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
