Question

A shoreline is eroding at a rate of 1\3 foot per months. how many feet are eroding per year

Answer 1

In the comments you said you meant every 3 months so... There are 4, 3 month periods in a year so 4 × 1/3 = 4/3 = 1 1/3 feet per year.

Answer 2

It erodes 1/3 every 3 months. There are 4- 3 months in a year.
1/3 x 4= 4/3 ft
4/3 ft a year