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Let original diameter be d that is 2r
new diameter=3/4d or 3/2r
original CSA 4*pi*r^2
=4*pi*4r^2=16*pi*r^2
new CSA=4*pi*9/4r^2
=9*pi*r^2
decrease=7*pi*r^2
decrease℅=(7*pi*r^2/16*pi*r^2)*100
=43.75℅

Let original diameter be 2x then radius =x c.s.a=4×pi×r^2 decreased diameter of sphere=2x-25% of 2x =2×-x/2=3/2x decreased radius=3/4x so,decreased c.s.a= 4×pi(3/4x)^2 =9/4×pi×x^2 decrease in area= 4×pi×x^2-9/4×pi×x^2 =7/4×pi×x^2 so, decreased percentage in c.s.a=7/4×pi×x^2/4×pi×x^2×100%=7/16×100%=43.75%