we have the fractions 8/15 and 5/8

if all you want to know is which is the larger and which is the smaller, you simply cross multiply
*upwards*, put the products above the fractions, * *and compare your products:

8x8=64 and 15x5=75 and 64<75, therefore 8/15<5/8

try another example: compare 11/13 and 7/9

9x11=99 and 13x7=91, 99>91

11/13>7/9

compare 11/12 and 13/15

15x11=165 and 12x13=156

165>156

11/12>13/15

if the products are equal, then the fractions are equal

don't forget: cross multiply **upwards** and put the products above the fractions;

mathematically what you are doing is using the product of the denominators as your common denominator and writing the fractions as equal fractions with this common denominator but without doing all of the work; this is useful if you are taking a test and have a limited amount of time