The unit fraction $\frac{1}{3}$
represents the space between the tick marks on the number line. Write the addition expression being modeled. Then find the sum.

A number line with two arrows on top of it. It has one tick mark to the left of 0 and five tick marks to the right of 0. One arrow begins at 0 and ends at the fifth tick mark to the right of 0. Another arrow begins at the fifth tick mark to the right of 0 and ends at the first tick mark to the right of 0.