The first thing is to calculate what she used for the card that would be:
20 - 17.9 = 2.1
Which means that she spent a total of $ 2.1.
They tell us that the value of the minute at long distance is 0.14 dollars, to know the time in minutes that it took, speaking, we must divide the value spent by the value of each minute, like this:
2.1 / 0.14 = 15
Which means it took 15 minutes for the only call he made.
