I prefer to give units on all the variables and make sure the units cancel.
Time (hr) = Distance (miles) / Speed (miles/hr)
= Distance (miles) * (1/Speed) (hr/miles)
= Distance/Speed (miles*hr/miles)
= Distance/Speed (hr)
We need hrs on the top.
Speed has hrs, but it is on the bottom.
So we flip Speed to put hrs on the top.

Famous Google job question.
How many piano tuners in America.
Just multiply these together with some flipped
so that everything cancels. 

Ie we want _____ (piano tuners / America)
I will make up useful numbers.
300000 (people/America)
100 (houses/piano)
5 (people/house)
1/5 (tuning)/((piano)(year))
8 (tunings)/((tuner)(week))
52 (weeks/year)



Ie we want _____ (piano tuners / America)
= 300000 (people/America)
x 1/5 (house/people)
x 1/100 (piano/houses)
x 1/5 (tuning)/((piano)(year))
x 1/52 (year/weeks)
x 1/8 ((tuner)(week))/(tunings)