To make them have a common denominator, you have to multiply. Since 5/6 already has a denominator that 3 can go into, all you have to do with 2/3 is multiply by 2. Whatever you do to the numerator you have to do with the denominator. So 2 x 2 = 4 and 3 x 2 = 6. The new pair of fractions would be 4/6 and 5/6