A linear equation takes the following form:
Y = mX + b
Y and X are Y and X, and m is the slope of the line, and b is the Y-intercept (the place where the line meets the Y axis).
For this problem, we can say that Y will be our cost-axis -- which is nice, as the cost goes up, the Y will go higher. X will be our time axis -- as we spend more time, we move further to the right. Nice!
Our initial cost will be b -- this is what our cost (Y) is where our time (X) is zero. $1.The slope is .25, this is what we multiply times X (time) to se how much we are spending.
Cost = $.25(time) + $1
For this problem:
Cost = $.25(180 minutes) + $1
Cost = $45 + $1
Cost = $46
All linear word problems are about like this one. It's just a matter of matching all the variables to the info you are given.