If you have a missing numerator (top number), divide the new denominator (bottom number) by the original denominator and multiply this by the numerator.
If you have a missing denominator, divide the new numerator by the original numerator and multiply this by the denominator.
examples:
Dividing denominators: 6 ÷ 2 = 3
Multiplying the numerator: 1 x 3 = 3
⇒ 1/2 = 3/6
Dividing numerators: 3 ÷ 1 = 3
Multiplying the denominator: 4 x 3 = 12
⇒ 1/4 = 3/12
Dividing denominators: 4 ÷ 8 = 1/2
Multiplying the numerator: 6 x 1/2 = 3
⇒ 6/8 = 3/4
Dividing numerators: 2 ÷ 3 = 2/3
Multiplying the denominator: 6 x 2/3 = 4
⇒ 3/6 = 2/4