Alright, lets get started.
Sophie has 5 pieces of string that are 4 yards each.
1 yards = 3 feet,
So, 4 yards = feet
Total number of string she has = 5 piece
So, total length she has = feet
It means she had 60 feet string in starting.
Now, She uses 3 pieces that are each 3 feet long, means
She uses = feet
So, remaining string length will be = feet
Means 51 feet string does Sophie have. : Answer
Hope it will help :)