"Can you express the fraction $55\over 108$ as the sum of two unit fractions$?$"
I figured out that ${1\over 54} + {1\over 2} = {56\over 108}$ but I could not figure out a way to get $55\over 108$ as a sum of two unit fractions.
How about $\frac 12 + \frac 1{108}$? Finding them in the general case can be difficult. You can look up Egyptian fractions to find ways to express rationals as unit fractions, but often finding the shortest expansion is not easy.