If the mean of the frequency distribution

Class	0-10	10-20	20-30	30-40	40-50
Frequency	2	3	$$x$$	5	4

is 28, then its variance is _______.

Mean = 28. Frequency distribution:

$$\sum f = 14 + x$$

$$\sum fx = 10 + 45 + 25x + 175 + 180 = 410 + 25x$$

Mean: $$\frac{410 + 25x}{14 + x} = 28$$

$$410 + 25x = 392 + 28x$$

$$18 = 3x$$, $$x = 6$$

$$\sum f = 20$$, $$\sum fx = 560$$

$$\sum fx^2 = 2(25) + 3(225) + 6(625) + 5(1225) + 4(2025) = 50 + 675 + 3750 + 6125 + 8100 = 18700$$

Variance = $$\frac{\sum fx^2}{\sum f} - \left(\frac{\sum fx}{\sum f}\right)^2 = \frac{18700}{20} - 784 = 935 - 784 = 151$$

The variance is 151.