You would not be incorrect in saying 1.66 moles. They might have been thinking one-third mole was added each time; so it is adding 0.333333 (repeating) which rounds up to 1.67. However, if it is exactly 0.33 moles (and not one-third mole), then you are correct (1.66 moles). Hopefully, that is an insignificant difference in doing these problems.