Q.(c) If the sum and arithmetic mean of several observations are 133 and 19 respectively, then find the number of observations.

The Arithmetic Mean: Finding the Centre of a Story

Imagine you and your friends are comparing pocket money. One friend gets ₹500, another gets ₹200, a third gets ₹800, and you get ₹300. If someone asked, "What is the typical pocket money in this group?" you wouldn't list all four amounts — you'd want a single number that represents the group fairly. That instinct — to find a middle ground, a representative value — is exactly what the arithmetic mean captures.

The arithmetic mean is the most common way of answering the question: "If everything were shared equally, what would each person get?" It is the balance point of a set of numbers. In everyday language, people call it the "average," though statisticians use "average" as a broader family that includes the mean, median, and mode.

The Precise Meaning

The arithmetic mean is calculated by taking the sum of all the values in a group and then dividing that total by the number of values. It answers the question: "What single value, if repeated for every item, would give the same total as the actual data?"

Think of it as a redistribution. If you had a pile of money equal to the total of all your friends' pocket money, and you wanted everyone to walk away with the same amount, the arithmetic mean is exactly that equal share. It is the number that makes the "total" stay the same while smoothing out the differences.

Why It Matters

The arithmetic mean is the foundation of almost all statistical thinking. Here is why it is so important:

• It gives a single reference point. Instead of describing a whole list of numbers, you can say "the average income in this neighbourhood is ₹40,000 per month." That one number summarises a complex reality.
• It allows comparison. You can compare the average test score of one class with another, or the average rainfall in one city with another. Without the mean, you would be comparing entire lists — which is impractical.
• It is the basis for further analysis. Many advanced statistical tools — like standard deviation, correlation, and regression — all start from the arithmetic mean. It is the anchor point from which we measure variation and relationships.
• It is used in everyday decision-making. Businesses calculate average sales per day to plan inventory. Governments use average income to set policies. Even your own grade point average is an arithmetic mean of your subject marks.

A Few Key Points to Remember …