The gauge of an oil tank indicates that it was 1/7 full. After 240 gallons were added, the gauge indicated that the tank was now 4/7 full. How many gallons does the tank hold? (Assuming that the gauge is accurate.) Here's the solution: Start with 3 is to 7 as 240 is to full. That is because we increased the volume by 37 from 1/7 to get to 4/7 by adding 240 gallons. How much would remain to be filled if we had added only 160 gallons when the tank was 3/7 full?

Question

The gauge of an oil tank indicates that it was 1/7 full.  After 240 gallons were added, the gauge indicated that the tank was now  4/7 full.  How many gallons does the tank hold?  (Assuming that the gauge is accurate.)  Here's the solution: Start with 3 is to 7 as 240 is to full.  That is because we increased the volume by 37 from 1/7 to get to 4/7 by adding 240 gallons.       How much would remain to be filled if we had added only 160 gallons when the tank was 3/7 full?