A man's pension on retirement from service is equal to half the average salary during the last $3$ years of his service. His salary from 1-1-1983 is $\text{₹}380$ per month, with increment of $\text{₹}40$ due on 1-10-1983, 1-10-1984 and 1-10-1985. If he retires on 1-1-1986, what pension does he draw per month?

The retirement date is 1-1-1986. The pension is based on the average salary during the last $3$ years ($36$ months) of service, which spans from 1-1-1983 to 31-12-1985.
Let's trace the monthly salary over these $36$ months:

*From 1-1-1983 to 30-9-1983 ($9$ months):**

$$\text{Salary}=\text{₹}380\text{/month}$$

$$\text{Total Earnings}=380\times 9=\text{₹}3420$$

*From 1-10-1983 to 30-9-1984 ($12$ months):**
An increment of $\text{₹}40$ is added.

$$\text{Salary}=380+40=\text{₹}420\text{/month}$$

$$\text{Total Earnings}=420\times 12=\text{₹}5040$$

*From 1-10-1984 to 30-9-1985 ($12$ months):**
Another increment of $\text{₹}40$ is added.

$$\text{Salary}=420+40=\text{₹}460\text{/month}$$

$$\text{Total Earnings}=460\times 12=\text{₹}5520$$

*From 1-10-1985 to 31-12-1985 ($3$ months):**
Another increment of $\text{₹}40$ is added.

$$\text{Salary}=460+40=\text{₹}500\text{/month}$$

$$\text{Total Earnings}=500\times 3=\text{₹}1500$$

Now, let's calculate the total salary over the $36$ months:

$$\text{Total Salary}=3420+5040+5520+1500=\text{₹}15480$$

Average monthly salary during this period:

$$\text{Average Monthly Salary}=\frac{15480}{36}=\text{₹}430$$

The pension draws half of this average monthly salary:

$$\text{Pension}=\frac{430}{2}=\text{₹}215$$