a 12ft street light is making an 18ft shadow. at the same time a nearby building is making a 72ft shadow. how tall is the buildi
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If we rewrite this equation, it would look like this: 
, which is the equation for a straight line with a slope of -4/3 and a y-intercept of 20. The y-intercept exists when x = 0. "x" represents the number of minutes he drives, "y" represents the number of laps he drives. Slope is rise/run, or y/x, which gives us that he drives 4 laps in 3 minutes (the slope of our line). The y-intercept, when x = 0, tells us that when he has driven 0 minutes, or hasn't driven at all yet, he still has 20 laps to go. When y = 0, the x-intercept is 
, and 
. Multiply both sides by 3 to get that -60 = -4x and that means that x is 15. This means that from start to finish, it takes him 15 minutes to drive the 20 laps.