Activity time is not calculated according to confirmed qty

Experts,

When we are confirming the production orders by CO11 activity time is not calculated according to confirmed quantity.

For Example:-

I have a order of 50 qty which tak machining time of 500 min..

If i am confirming the Production order with 10 qty it should calculate maching time as 100 mins.But system still shows machining time as 500 min.

Please suggest how to map[ it.