Physics pendullum numerical question---Please explain?

a simple pendulum bob of m 10gm starts its oscillation from the right extreme position .Before it starts oscillation , the height of the bob at the extreme position is 10cm with respect to its mean position . After every one complete oscillation, the height of the bob at its extreme position decreases by 1cm.Calculate the work done by the air friction during every oscillation . ( take g=10m/s square)