To understand how the multiplier works, please check out the following example:

If a girl has 4x, 5x, and 3x multipliers, the total multiplier will be 60x (4 times 5 times 3).

As a general rule, you want to increase the multiplier as much as possible for the girls you usually hire in order to increase the coin production. The higher the personal multiplier, the more coins you get from that girl.

Increasing the personal multiplier can be done by purchasing and upgrading certain Decorations. For more info about the Decorations, you can check the dedicated FAQ section.