Calculate the remaining units to be billed at the second rate if total consumption is 300 units and the first 220 units are billed at 50 FCFA per unit.

Total units consumed $= 300$ units.

The first 220 units are billed at 50 FCFA per unit.

The remaining units are the total units minus the units billed at the first rate.
Remaining units $= \text{Total units consumed} - \text{Units billed at first rate}$
Remaining units $= 300 - 220$
Remaining units $= 80$ units

These 80 units are the "rest" that are billed at 79 FCFA per unit.

The number of units to be paid at 79 FCFA is $\boxed{\text{80 units}}$.