Explanation:
always find the quantity of 100%.
then
1% = 100%/100
then calculate the absolute quantity change.
then divide the quantity change by 1% to see how many % fit into it.
and that is your answer.
so, if the original quantity is q1, the new quantity is q2, then the % of quantity change is
|q1 - q2| / (q1/100)
or
100 × |q1 - q2| / q1
where || means the absolute value.