The percentage change is given by
... ((new value) - (old value))/(old value) × 100%
If the original fraction is 100/100, we can decrease its numerator by 50% by making it be 100-50 = 50. We can decrease its denominator by 25% by making it be 100-25 = 75. Then your percentage is ...
[tex]\displaystyle\frac{\text{(new value)-(old value)}}{\text{(old value)}}\times 100\%=\frac{\left(\frac{100-50}{100-25}\right)-\left(\frac{100}{100}\right)}{\left(\frac{100}{100}\right)}\times 100\%\\\\=\left(\frac{50}{75}-\frac{75}{75}\right)\times 100\%=-\frac{25}{75}\times 100\%\\\\=-\frac{1}{3}\times 100\%=-33.\overline{3}\%[/tex]
Note that we have taken advantage of the fact that 100/100 = 1 = 75/75 to simplify the expression dramatically.
The percentage change is -33.3%
