Review of the proportion and cross products.

The following problems can be solved using cross products:

I- If 3 hambergers cost $6. How much do 5 hambergers cost ?

We re-arrange the problem so we can apply the cross products:

3 hambergers cost $6      (1)
5 hambergers cost $x      (2)

Cross product:

3 · x = 5 · 6
3x = 30
x= 30/3
x = 10

So 5 hamberger cost $10.

Remember to re-arrange (1) and (2) in the way that the units must be the same: hamberger to hamberger ; $ to $. If the units are not the same, we have to change the numbers to get the same units. For example:

---------o0o---------

II- If it takes us 1hr 20 min to walk 5 miles, how long will it take us to walk 6 miles ?

We re-arrange the problem so we can apply the cross products:

1hr 20 min to walk 5 miles.      (3)
x min to walk 6 miles.            (4)

We should change both (3) and (4) to have the same units, minutes and miles:

80 min to walk 5 miles.       (5)
x min to walk 6 miles.         (6)

Cross product:

5 · x = 80 · 6
5x = 480
x= 480/5
x = 96

So it takes 96 minutes or 1hr 36 min to walk 6 miles.