Answer:

To find the missing frequency (f) when the mean is 18, we can use the formula for the mean:

Mean = (Sum of all values) / (Total number of values)

In this case, the mean is given as 18. We can calculate the sum of all values by multiplying each frequency with its corresponding midpoint. The midpoint of each class interval can be calculated by taking the average of the lower and upper limits.

Using this information, we can set up the equation:

18 = [(12 + 14) * 7 + (14 + 16) * 6 + (16 + 18) * f + (18 + 20) * 13 + (20 + 22) * 20 + (22 + 24) * 5 + (24 + 26) * 4] / (7 + 6 + 1 + 13 + 20 + 5 + 4)

Simplifying the equation, we have:

18 = [(26 * 7) + (30 * 6) + (34 * f) + (38 * 13) + (42 * 20) + (46 * 5) + (50 * 4)] / 56

Multiplying both sides by 56, we get:

1008 = 182 + 180 + 34f + 494 + 840 + 230 + 200

Combining like terms, we have:

1008 = 2136 + 34f

Subtracting 2136 from both sides, we get:

-1128 = 34f

Dividing both sides by 34, we find:

f = -1128 / 34

Simplifying further, we have:

f ≈ -33.18

Since the number of patients cannot be negative or fractional, it is not possible to have a missing frequency in this case. Please double-check the given data or the calculation process.

Step-by-step explanation:

steps are given in detail above