Pipe A will fill a tank in 6 hours. Pipe B will fill the same tank in 4 hours. Pipe C will fill the tank in the same number of hours that it will take Pipes A and B working together to fill the tank. What fraction of the tank will be filled if all three of the pipes work together for one hour?
In one hour, pipe A fills up 1/6 of the tank and pipe B fills up 1/4 of the tank. Therefore pipe A and pipe B working together would fill up 1/6+1/4 = 5/12. Pipe C fills up the tank in the same time so it also fills up 5/12 of the tank. Therefore A+B+C = 5/6 in one hour.