For this you will need to remember how to add fractions and have a basic understanding of prime numbers.

We are given the denominators of four fractions as 4, 6, 9 and 10. All we know about the numerators is that the fractions are irreducible so that, for example, the numerator for 4 cannot be 2.

Find the prime factorization of the denominators to show that the least common denominator is 180. Then show that when the four fractions are added the resulting sum, with 180 as the denominator, is irreducible.