When a fixed amount is withdrawn at the end of each quarter, interest on drawings will be calculated for _______

The formula for average period is:
$$\text{Average Period}=\frac{\text{Months left after first drawing}+\text{Months left after last drawing}}{2}$$
For withdrawals at the end of each quarter, the first drawing is made at the end of the 1st quarter (9 months left in the year).The last drawing is made at the end of the 4th quarter (0 months left).
$$\text{Average Period}=\frac{9+0}{2}=4.5\text{ months}=4\frac{1}{2}\text{ months}$$