A water tank is 1/8 full. The tank is 3/4 full when 42 gallons are added to the tank. How much water was originally in the tank? (I rewrote it, your wording was messed up)
3/4 = 6/8
1/8 + 6/8 = 7/8 = 42 gallons
42 divided by 8 = 5 1/4
So, the answer is 5 1/4. The tank originally 5.25 gallons of water.