Given Information:
- Mean pocket allowance = Rs 18

Assumptions:
- The data represents a frequency distribution.
- The distribution is symmetric.
- The missing frequency is denoted by 'f'.

Approach:
To find the missing frequency 'f', we need to use the formula for the mean of a frequency distribution. The formula is given as:

Mean = Sum of (Frequency * Value) / Total Frequency

We can rearrange this formula to find the missing frequency 'f':

f = (Mean * Total Frequency) - Sum of (Frequency * Value)

Calculation:
Let's assume the given data represents the frequency distribution table as follows:

| Pocket Allowance (in Rs) | Frequency |
|--------------------------|-----------|
| 10 | 5 |
| 15 | 8 |
| 20 | 10 |
| 25 | f |
| 30 | 12 |
| 35 | 15 |

We can calculate the sum of (Frequency * Value) using the given data:

Sum of (Frequency * Value) = (10 * 5) + (15 * 8) + (20 * 10) + (25 * f) + (30 * 12) + (35 * 15)

Now, we substitute the given values into the formula for the mean:

18 = Sum of (Frequency * Value) / Total Frequency

To find the missing frequency 'f', we rearrange the formula:

f = (Mean * Total Frequency) - Sum of (Frequency * Value)

Substituting the values into the formula, we get:

f = (18 * Total Frequency) - (Sum of (Frequency * Value))

Conclusion:
To find the missing frequency 'f', we need to substitute the values into the formula for the mean of a frequency distribution. By rearranging the formula, we can solve for the missing frequency 'f'.

F=20