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)

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)

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

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 !!

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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)