To find their common denominator, you must first find the lowest common multilple for 8 and 3.
8 × 3 = 24
24 is the lowest common multilple. Now you just have to make 24 the common denominator by multiplying the fractions like so:
[tex] \frac{5}{8} \times \frac{3}{3} = \frac{15}{24} \\ \frac{2}{3} \times \frac{8}{8} = \frac{16}{24} [/tex]
Now they both contain the same denominator, we can compare the equasions easier that way:
[tex] \frac{15}{24} < \frac{16}{24} [/tex]
If we use the original fractions, the answer is:
[tex] \frac{5}{8} < \frac{2}{3} [/tex]
