Diamond wheels should not be used for grinding steel components

If under a condition of plain turning the life of the cutting tool decreases by $$50\% $$ due to increase in the cutting velocity by $$20\% $$, then what is the $$\% $$ increase in tool life due to reduction in the cutting velocity by $$20%$$ from its original value.

While turning a $$C15$$ steel of $$160mm$$ diameter at $$215rpm,$$ $$2.5mm$$ depth of cut and feed of $$0.16mm/rev$$ by a tool of geometry $$0-10-8-9-15-75-0mm,$$ the following observations were made. Tangential component of the cutting force $$=500N,$$ axial component of the cutting force $$=200N,$$ chip thickness $$=0.48mm.$$ Determine the dynamic shear strength of the work piece material.