Question

What is the equation for a billing process that has a one-time fee of $100 and a monthly charge of $25?

Answer 1

For the time in months `t=0`, the value of the bill equals `$100`.
After `1` month, the amount charged increases in `$25`, to `$125`.

Lets put it on a `month -> bill` table:
`0->100`
`1 ->100 + 25`
`2->100 + 25 + 25 = 100 + 2*25`
`3->100 + 25 + 25 + 25 = 100 + 3*25`
`...`
`m->100 + 25*m`

The bill after `m` months is given by the equation
`B(m) = 100+25.m`//

Hope it helps.