This instructable is an entry to the Burning Questions Round 6.5 contest.
The fraction images were generated using this site
Step 1: Multiplying Whole Numbers
Step 2: Multiplying Fractions by Whole Numbers
Step 3: Multiplying Fractions Together
If necessary, remember to simplify your fraction!
Step 4: Cross-Cancelling
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)