Amy runs 6 miles in 45 minutes. at the same rate, how many minutes would she take to run 4 miles?

That would be (4/6) of 45 minutes, or 180/6 minutes, or 30 minutes.

You could also find her rate:  6 miles / 45 minutes, or 8 miles / 60 minutes.

Write an equation of ratios:  

8 mi            4 mi
=
60 min            x

Solving for x:  8x = 240; x = 30 minutes

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