ok so im not good with words and math, but i am going to try my best to explain it. first you will take 45/60 which is the 45 miles per hour. next you will put it equal to 40/x. x is how many minutes. they are set equal to each other because they are the same ratio. (i honestly dont know why you cross multiply, but you just do). you then multiply 40*60 and 45*x. this gives you 2400=45x. you then divide both sides by 45 and get 53 1/3 minutes. or 53 minutes and 20 seconds
