Multiplicative

"Multiplicative" (or simply Multi in player abbrieviations) is a property that can be applied on any bloon. When applied a bloon will release 4x as many bloons, and they will have the Multiplicative property too. They can be identified by the bunny ears on top of the bloon (or in front of an MOAB-class bloon). Low level bloons can easily be dealt with, but even 1 increased level of a Multi bloon makes it exponentially harder to destroy. Regen Multi bloons only appear in Impossible because of the difficulty of destroying all bloons before the others can regenerate into 2 or more of the same Multi bloon. Also, power stripping abilities such as Reactor do nothing to Multiplicative bloons.

BTD5 bloons with Multi
Red: RBE 1

Blue: RBE 5

Green: RBE 21

Yellow: RBE 85

Pink: RBE 341

Black/White: RBE 2,730 (Note that from here on Multi bloons have 8x the previous bloon inside them. 2*4=8. Comparison: this is roughly 4-5x the RBE of a regular MOAB)

Zebra/Lead: RBE 21,841 (Comparison: quite a bit larger than the RBE of 1 ZOMG)

Rainbow: RBE 174,729

Ceramic: RBE 1,397,833

MOAB: RBE 22,365,528 (x16)

BFB: RBE 357,849,148

ZOMG: RBE 5,725,590,368

BTD6 bloons with Multi
Purple: RBE 2,730

DDT: RBE 22,365,728

BAD: RBE 46,073,111,680

Strategy
Brute force and fast-firing towers are of little use. It would take a virtually infinite number of shots to destroy high level Multi bloons.

Snipers are definitely a must-buy, due to their ability to destroy many layers at once. A 3-0 sniper monkey can destroy an entire Multi Ceramic (which has more RBE than 83 ZOMGs combined) with one shot, of course, if not for the "assistance" of other towers. Remember, the key is to completely destroy multiple layers at once before they have the ability to multiply.

Extremely penetrative towers are also extremely useful. Ray of Doom spam could completely take out Multi bloons with little difficulty.

Aura towers definitely make the paramount lists. Ice towers and Reactors destroy a group of bloons at once, smashing through Multi bloons as though the "Multi" didn't exist.