**Enrollment at Marvin Primary School Term One**

The initial enrollment at Marvin Primary School for term one was 400 pupils. The enrollment increased by 12½. We need to find the new enrollment at the school.

**Understanding the Increase**

To understand the increase in enrollment, let's first convert the given increase of 12½ into a decimal.

12½ can be written as 12.5 in decimal form.

**Calculating the Increase**

To calculate the increase, we need to find 12.5% of the initial enrollment.

12.5% of 400 can be calculated as:
(12.5/100) * 400 = 0.125 * 400 = 50

Therefore, the increase in enrollment is 50 pupils.

**Finding the New Enrollment**

To find the new enrollment at Marvin Primary School, we need to add the increase to the initial enrollment.

New Enrollment = Initial Enrollment + Increase
New Enrollment = 400 + 50
New Enrollment = 450

Therefore, the new enrollment at Marvin Primary School is 450 pupils.

**Explanation**

In this problem, we were given the initial enrollment at Marvin Primary School for term one as 400 pupils. We were also given an increase of 12½, which we converted to 12.5 in decimal form.

To calculate the increase, we found 12.5% of the initial enrollment, which is equal to 50 pupils.

Finally, we added the increase to the initial enrollment to find the new enrollment at Marvin Primary School, which is 450 pupils.