**Problem**:

On Monday, a person mailed 8 packages weighing an average (arithmetic mean) of 12 3/8 pounds, and on Tuesday, 4 packages weighing an average of 15 1/4 pounds. What was the average weight, in pounds, of all the packages the person mailed on both days?

**Solution**:

The average weight of the packages is the total weight divided by the number of packages:

Avg. = Total/#of items, or

Total = Avg. * # of items

Using this form of the average equation, the total weight for all 12 packages is:

8*12⅜ + 4*15⅟_{4} = 99 + 61 = 160.

The number of packages is 8 + 4 = 12.

Thus, the average weight is 160 / 12 = 13⅓.

**Strategy**:

With twice as many packages with average 12⅜ as 15⅟_{4}, the answer will be ⅓ of the distance from 12⅜ to 15⅟_{4} (think of it as two packages weighing 12⅜ and one weighing 15⅟_{4}). Also, one can immediately eliminate the choices weighing more than 15⅟_{4} as the averages must be less than that.

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