MATHS! please~

Given that y varies inversely as x,
1a) find the percentage decrease in y if x increases by25%,
設 y = k/x
若 x’ = (1+25%)x
y ‘ = k/x’
y ‘ = k/[(1+25%)x]
y ‘ = (k/x)(1/1.25)
y ‘ = 0.8y
所以減少了
(y – y’)/y = (y – 0.8y)/y
= 20%

b) find the percentage change in x if y increases by10%.
y” = k/x”
y” = (1+10%)y = 1.1y
x” = k/1.1y
= x (1/1.1)
= 0.9091x
x 減少了
(x – x”)/x
= (x – 0.9091x)/x
= 9.091%
(亦即是減少了 (100/11)%)