It’s Valentine’s day and lots of couples want to go for a ride on the Ferris wheel of love. The wheel of love has ten seats and each seat can fit one couple. Each minute a seat passes by the entrance/exit stage. The wheel starts functioning at 8pm for half an hour – do you know how many couples took a ride?

Answer: 30 couples.

Justification:

You can follow a systematic approach to tackle this kind of problems.

Construct a chain of ratios whose units cancel out to yield to a result with the right units of the answer you are looking for.

In this case you want to know the number of couples that took the ride in 30 minutes.

The input data are:

1 couple / seat

1 seat / minute

Then, by multiplying those two data you get: [1 couple / seat] * [1 seat / minute], which, treating the units appropiately yields to 1 couple / minute.

Next, you realize that when you muliply 1 couple / minute times the number of minutes you get the number of couples that took the ride in that time.

So, the answer is: 1 couple / minute * 30 minute = 30 couples (the minutes cancel out because they appear in the denominator and numerator).

So, when you work with ratios and proportions, the use of units helps you a lot to figure out the way to get the answer.