Remember that 1 yard = 3 feet. So if Cole has string that is 5 yards each, each string should be 15 feet long (5 x 3). Since each string is 15 feet long and Cole has 8 strings, Cole should have 120 feet of string (15 x 8).
Cole now uses 4 pieces that are each 5 feet long, so 4 x 5 = 20 feet used.
What's 120 - 20? That should give you your answer.