Environmental biology

Hello guys,

I have an exam in the morning and i really need your help with the following problem. It would mean a lot to me!

If 20 ml of sludge with a methane production rate of 10 umol/ml
sludge/day is incubated at 25 degrees C in a 120 ml bottle, how long would it take for the headspace of the bottle to be 10% methane?

1mol gas = 25L at 25 degrees C. Pressure in the bottle does not change.