Chris has a 3.5 feet long board of wood. he cuts out 4 pieces that are each 2/3 foot long.

I'm assuming you want to know the amount of feet left?

Chris starts with 7/2 feet of wood and removes 8/3. We put both under a common denominator (sixths), getting 21/6 feet of wood and removing 16/6.

Doing the subtraction, we are left with 21/6-16/6=5/6 feet of wood.