One-third of the result of three times a number that is increased by 12

Find the missing dimension extra points

Atrain leaves little? rock, arkansas, and travels north at 90 kilometers per hour. another train leaves at the same time and travels south at 80 kilometers per hour. how long will it take before they are 680 kilometers? apart?

If you have 12 feet of string and you cut it into equal length of 5 inches each how much string will be left