Question

A student walks at a brisk 3.50 mi/hr. Calculate the student's speed in yards/minute.

I have 4 questions just like this one. Could you tell me step by step on how to do the double conversions? Thanks!

Answer 1

You do them step by step. I use units I have memorized; sometimes that means going around the house more times to get where I'm going but it keeps me from memorizing a book of conversion factors.

3.50 mi/hr to yards/min.
I remember there are 5,280 feet/mi
There are 3 feet/yard

I remember there are 60 min in an hour.

3.50 mi/hr x (5,280 feet/mi) x (1 yd/3 feet) = ? yards/hr. Note I worked ONLY with the mile part and left the hr to another time. Now I can do that.

? yards/hr x (1 hr/60 min) = ? yds/min = ?
How do you know to multiply or divide by those conversion factors I remember. You multiply in such a way so the unit you don't want cancel and the unit you want to convert to stays. See the first factor
3.50 mi/hr x (5,280 feet/mile)= ?
I know to do (5,280 feet/mile) BECAUSE the mile unit cancels and leaves feet and that's where I want to go. If I had made it 3.50 mi/hr (1 mi/5,280 feet) the units of that would be (miles)^2/feet hr and that isn't what I want.
Note: You can always go to google and type in 3.50 miles/hour to and hit the enter button. I think 102.7 pops up.