A pipe fills a tank in 6 hours while an outlet empties it in 8 hours. With both open the tank fills in:

1. The inlet fills one sixth of the tank each hour. 2. The outlet removes one eighth of the tank each hour. 3. The net filling rate is 1/6 - 1/8. 4. Over a common denominator of 24 that is 4/24 - 3/24 = 1/24 per hour. 5. So the tank fills in 24 hours. Adding the rates instead of subtracting would give about 3.4 hours, ignoring that one pipe works against the other. _Source: syllabus Maths, Ch 11 'Direct and Inverse Proportions', S11.3 Inverse Proportion_