You have to find the slope first using m=y2-y1 divided by x2-x1. After you found the slope of the line, you use one of the points to plug it into the point-slope form which is y-y1=m(x-x1). After you have done that, you would have convert this equation into slope-intercept form which is y=mx + b. In order to convert it, you have to multiply m with x and x1. Then you would have to get rid of y1 by doing the opposite of what y1 is. Finally, you would take the opposite of y1 and add it to the other side of the equation.
Your equation would be 2.50x + 5. Then subtract 5 from 20. then take 15 and divide it by 2.50. Your answer would be 6. you could ride 6 miles! Good Luck!