There were 2/3 as many children as adults on a bus. When 12 children got off the bus and 12 adults boarded, the number of children became 1/9 the number of adults on the bus. How many adults were on the bus at first?
c = 2/3 a
c -12 = 1/9 (a+12) sub in the red equation
2/3 a - 12 = 1/9a + 12/9
a = 24 originally