Since your question doesn't have values, I will use symbols. For the monthly base fee, we will use F. For the charge per minute we will use C, and for the number of minutes we will use m. The total bill will be P. So we get:

P=F+mC

If the smallest bill was Q, then `Pgt=Q`

Since P = F+mC, then the equation becomes:

`F+mCgt=Q`

the number of minutes used can be solved by isolating m.

`mgt=(Q-F)/C`

Let's try it with some numbers: F = $20. C = $0.05/min, Q = $35

`mgt=($35-$20)/($0.05/\text(min))rArrmgt=300\text(min)`

Hope that helps!