Water changes & Temperature Fluctuations

Hey everyone

So now that my tank is cycled, i have another question before i add fish
regarding water temperature when changing water.

When i changed my water i filled my 20L jug with tap water (very cold)
I poured 2L of that into a kettle and boiled it and poured it back in to
raise the temp to closer to my tank temperature (but it was still a
little colder). I didn't get too worried with it being that i have no
fish, so needless to say, even with boiling some water the water in the
tank did drop in temperature by 4 degrees.

My question is, i need some formulas to figure out things like

"if i have 'a' litres of water @ 'x' degrees, and I add 'b' litres of
water @ 'y' degrees, then a+b = c litres at 'z' degrees."

OR

"if i have 'a' litres of water @ 'x' degress, how much water (b) at 'y'
degrees do i have to add to make a+b = z degrees."

I am sure i took some of this stuff in school some time, but damned if i
can remember any of it. Just one formula is good, i can re-work it to
find what i need (i remember that much! lol). Even a google search
term would be great.

Thanks

James