What is the percentage increased in the volume of a gas when the pressure is reduced to half at constant temperature?

100% increase.

Explanation:

At constant temperature we have the relation

P1V1 = P2V2 where P and V are pressures and volumes, so here we have, say

P1 = 10, P2 = 1/2 * 10 = 5, V1 = 10, V2 = ?

10 * 10 = 5 * V2

V2 = 100/5 = 20.

The original volume was 10 so the volume has doubled.