Instructables

Multiplying Fractions

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Picture of Multiplying Fractions
How to multiply fractions!
This instructable is an entry to the Burning Questions Round 6.5 contest.
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Step 2: Multiplying fractions by whole numbers

The theory is the same for multiplying fractions by whole numbers(See image)

Step 4: Cross-Cancelling

Cross cancelling is basically simplifying before you multiply, but, don't just simplify the fractions the normal way, instead cross-cancel. Cross-cancelling also makes it easier to multiply if using large numbers.
To cross-cancel the the multiplication 81/21 X 49/27,(See first image) try and divide the diagonals by the same number, e.g.
21 and 49 can both divide by 7, so we replace them with 3 and 7 respectively(See second image),
then we cancel the other diagonal, 81 and 27 can both be divided by 9, so we replace them with 9 and 3(See third image), then do the multiplication as in step 3. Remember that it doesn't matter if you can only cross-cancel in 1 direction.
The final answer to 81/21 X 49/27 is the improper fraction 63/9, in a mixed number that's 7/1 or just 7(See fourth image)
dolphinlms2 years ago
This could have been cross cancelled two more times. The 9 & 3 (either denominator!) could be cross cancelled so you would have either 3/1 x 7/3 = 1/1 x 7/1 = 7 OR 3/3 x 7/1 = 1 x 7 = 7. I review this with 6th graders by writing a ridiculously long multiplication problem to demonstrate how much you can simplify a problem.
wow, education is very important !!
I'm WITH YOU ON THIS ONE!!! EDUCATION IS LIKE PIZZA !!! YOU CAN'T GET ENOUGH!!!
but i think you can cancel from the top and the bottom of the same fraction because it is just the same thing as a diagonal (it's one of the properties of multiplication)