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Search the Web  ## Ratio, Cross-Multiplication - Level 2.

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1- Do the following maths.
2- Hit Submit

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:

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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.

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Do the following problems:

A- If 12 galons of milk cost \$ 48, how much it costs to buy 5 galons of milk?

Guide:
12 galons of milk cost \$ 48
5 galons of milk cost \$x

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B- If it takes 1 hrs 30 minutes to produce 9 meters of fabric, how many minutes it take to produce 13 meters of fabric?

Guide:
9 meters of fabric take 1 hrs 30 minutes (or 90 minutes)
13 meters of fabric take x minutes