the answer=x·（1+10%）·（1-10%）
a number x is increased by 10% ：y=x·（1+10%）,now the number becomes y.
then decreased by 10% :the answer=y·(1-10%)=x·（1+10%）·（1-10%）
because the latter 10% is for y,not for x.
因为后来下降的10%,是y的10%,不是x的10%.
the answer=x·（1+10%）·（1-10%）
a number x is increased by 10% ：y=x·（1+10%）,now the number becomes y.
then decreased by 10% :the answer=y·(1-10%)=x·（1+10%）·（1-10%）
because the latter 10% is for y,not for x.
因为后来下降的10%,是y的10%,不是x的10%.