loading

Step 4: Cross-Cancelling

Picture of Cross-Cancelling
4a.tif
4b.tif
4c.tif
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)
 
AlliP11 months ago

how do you multiply them if neither number can go into each other when cross reducing, would you just keep the same number?

dolphinlms3 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.