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Question

The average rainfall over a given place during the three-year period of 2003-2005 was 65 cm. During the three-year period 2002-2004 the average rainfall was 63 cm. The actual rainfall during 2005 was 60 cm. What was the rainfall in 2002?

The correct answer is
54 cm

This problem involves calculating the rainfall for a specific year using average rainfall data over overlapping periods.

Calculating Total Rainfall for 2003-2005

The average rainfall for the three-year period 2003-2005 is given as 65 cm. The total rainfall for this period is calculated as:

Total Rainfall (2003-2005) = Average Rainfall $\times$ Number of Years

Total Rainfall (2003-2005) = $65 \text{ cm} \times 3 = 195 \text{ cm}$

Let $R_{2003}, R_{2004}, R_{2005}$ represent the rainfall in the years 2003, 2004, and 2005, respectively. So,

$R_{2003} + R_{2004} + R_{2005} = 195 \quad \text{(Equation 1)}$

Calculating Total Rainfall for 2002-2004

The average rainfall for the three-year period 2002-2004 is 63 cm. The total rainfall is:

Total Rainfall (2002-2004) = Average Rainfall $\times$ Number of Years

Total Rainfall (2002-2004) = $63 \text{ cm} \times 3 = 189 \text{ cm}$

Let $R_{2002}$ represent the rainfall in 2002. So,

$R_{2002} + R_{2003} + R_{2004} = 189 \quad \text{(Equation 2)}$

Determining 2002 Rainfall

We are given that the actual rainfall during 2005 was 60 cm. So, $R_{2005} = 60 \text{ cm}$.

Substitute the value of $R_{2005}$ into Equation 1:

$R_{2003} + R_{2004} + 60 = 195$

Subtracting 60 from both sides gives the combined rainfall for 2003 and 2004:

$R_{2003} + R_{2004} = 195 - 60$

$R_{2003} + R_{2004} = 135 \quad \text{(Equation 3)}$

Now, substitute the value of $(R_{2003} + R_{2004})$ from Equation 3 into Equation 2:

$R_{2002} + 135 = 189$

To find the rainfall in 2002, subtract 135 from both sides:

$R_{2002} = 189 - 135$

$R_{2002} = 54 \text{ cm}$

Therefore, the rainfall in 2002 was 54 cm.

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Important Questions from Average (Notes)

  1. The average age of a group of boys and girls is 18 years. If the average age of boys is 20 years and that of girls is 15 years, then what is the percentage of boys in the group?
  2. The average of 13 numbers is 8. If each number is multiplied by 7, then what will the new average be:
  3. There are 78 members in group A, 22 members in group B and 34 members in group C. All the members of these groups went to a restaurant. The average amounts spent on each member of groups A, B and C are ₹136, ₹383 and ₹457, respectively. The overall average amount (in ₹) spent per member is:
  4. If the average of p numbers is $q^2$ and that of q numbers is $p^2$, then the average of (p + q) numbers is :
  5. The average of 101 consecutive odd numbers is 303. Find the largest number.
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