When you have an exercise like 81/21 x 49/27 you make think: this is to difficult fo me. Now cross reducing fractions will help you to overcome this problem. Cross-cancelling will help you when multiplying a fraction.
Cross cancelling is basically simplifying fractions before you multiply. You don't simplify the fractions the normal way. Cross-cancelling makes it easier to multiply if using large numbers because it makes to numbers smaller.
To cross-cancel the multiplication 81/21 x 49/27 divide the diagonals by their greatest common factor. 21 and 49 can both divide by 7, so we replace them with 3 and 7 respectively, then we cancel the other diagonal, 81 and 27 can both be divided by 9, so we replace them with 9 and 3, 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.
Cross cancelling fractions or cross reducing fractions makes multiplying and dividing easier. Cross-canceling means that when multiplying fractions you can reduce the numerator of one fraction with the denominator of another.
For reducing fractions and for cross cancelling fractions when multiplying fractions it is handy when you know an easy way to find the greatest common factor. Learn more on greatest common factor.