Calculating drawing odds on the flop

[Eric P]

Can someone tell me the formula to doing odds combining the turn in the river. I know most the important ones but i'm not 100% how to do the calculations

For example. what are the odds to improve if you flop trips (banning straights and flushes)
I see it as:
you've seen 5 cards so 47 left in the deck, 7 ways to improve (board pairs or you quad) so 7/45 or 15%, then on the turn you have 10 outs so 10/45 or 22%. Do i simply add these percentages? If some one could explain this to me i would apprciate it. I had just been adding them but i want to make sure i'm right.