You know that we can Multiply the Numerator and denominator of a fraction by the same number and the value of the fraction does not change:
1/2 = (2 * 1) / ( 2 * 2) = 3/ 6 = 4/ 8 = ....... 50/100
This says that if we split a unit, first into 2 part, 2nd into 4 part, 3rd into 6 part, ...... , and into 100 part
1 of the first split, 2 of the 4 split, 3 of the 6 split, ..... , 50 of the 100 split
will all add up the same quantity.
Now:
if we have 1/2 = x / 6
We have 2 fraction equal, therefore, 6 divided by 2 = 3 , X has to be multiplied by 3 to keep the values of 2 fractions equal
to solve this equation:
1/2 = x/6
We multiply both sides by 6
6*1/2 = 6 *X / 6
Now divide both sides by 6
Simplify each fraction
X = 3
So , that is where the cross multiplication of 2 equal fraction comes into picture.
Multiply both fraction with LCM of the denominators, and then simplify each fraction to find the
value of the variable.