6. How many $3 \times 5$ rectangular pieces of cardboard can be cut from a $17 \times 22$ rectangular piece of cardboard, when the amount of waste is minimised?
6. 24 is the maximum number of $3 \times 5$ cards that can be cut, since the amount of waste is then 14 square units, less than the area of one $3 \times 5$ card. There do exist configurations with 24 pieces.