If you find a common denominator for both of the fractions you will be able to see clearly which is higher. Use 80 as the common denominator so multiply 4 (above 8) by 10 to make 40 and multiply the 4 (above 10) by 8 to make 32. This gives 40/80 and 32/80 so you can tell that 4/8 is higher.
