As a more general rule of thumb, imagine that the end of the track will pop each bloon that hits it, at the cost of one life. For example, let's say a black bloon makes it to the end of the track. You lose one life, popping the black layer, then two pink come out. They both pop, dealing one each, then two yellow bloons pop out. They pop, doing one damage a piece, and finishing with two greens. Same for blue and red. After all is said and done, you have taken 11 damage from that black bloon, even though the black bloon itself only did 1 damage, each of the ten remaining bloons the came out of it also did one damage, so you took 11 damage total. As you progress, it gets more and more complicated.