The workpart in a turning operation is 88 mm in diameter and 400 mm long. A feed rate of 0.25 mm/rev is used in the operation. If cutting speed = 3.5 m/s, the tool must be changed every 3 workparts; but if cutting speed = 2.5 m/s, the tool can be used to produce 20 pieces between tool changes. Determine the cutting speed that will allow the toll to be used for 50 parts between tool changes.