Equivalent Fractions · Hard problem

Co reduces $\dfrac{72}{108}$ to lowest terms in two steps: first dividing top and bottom by 4 to get $\dfrac{18}{27}$, then by 9 to get $\dfrac{2}{3}$. She could have done it in just ONE step. What is the largest number she could have divided 72 and 108 by right away?