General Idea:
% decrease [tex] =\frac{O-F}{O} * 100 [/tex] where O means original value, F means final value.
Applying the concept:
In our problem, original value is 6.5 kg and final value is 3.9 kg. Substituting this in the formula we get the percentage decrease.
% decrease [tex] =\frac{6.5-3.9}{6.5} *100=\frac{2.6}{6.5}*100= 0.4*100=40 [/tex]
Conclusion:
The percent decrease in the mass of the pumpkin is 40%