To find the decreased percentage,here's what we can do:
[tex] \frac{originl \: value - new \: value}{originl \: value} \times 100\%[/tex]
In this case,the eqaution would be:
[tex] \frac{280 - 210}{280} \times 100\% \\ = \frac{70}{280} \times 100\% \\ = \frac{1}{4} \times 100\% \\ = 25\%[/tex]
Hope it helps!