An electric pump can fill a 45-gallon tub in half an hour. at this rate, how long would it take to fill a 60-gallon tub?

You can cross multiply to find the answer. Start by setting up your equation:

45/30 = 60/x  (30 represents half an hour)
Next, cross multiply, 45 times x, and 60 times 30: 

45x = 1800  you must isolate the "x" value by using the opposite operation it is involved in. Here we have 45x (x is multiplied to 45) so we must divide 45, which will cancel itself out, leaving us with "x."

What you do on one side of the equation, you must do on the other, so divide 1800 by 45 as well, which will leave you with 40.

Working it out on paper, your equation should now look like this: x=40, which represents the amount of time it will take to fill a 60 gallon tube.

It will take 40 minutes to fill a 60 gallon tube. 

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Main idea on left, details on right