BillNye

I know there is a way but I can't seem to remember the equation for temperature change in a flow. Like if I have water that is 30 C with a rate of 30 liters per minute and another flow of water at 40C with a rate of 10 liters per minute, how could I calculate the final temperature of the mixture? I know there is an equation for this but I can't seem to remember. Anyone have any suggestions? Thank you.