Question

A cell phone company offers a contract which cost C, in dollars, of t minutes to telephoning is given by C=0.25(t-400)+57.95, where it is assumed that t is greater than and equal to 400 minutes. what times keep cost between $98.95 and $138.95?
For the cost to be between 98.95 and 138.95, the telephoning must be between ? minutes and ? minutes?

I don't even know where to start