The sum of deviations of n numbers from 10 and 20 are p and q respectively. If (p - q)2 = 10000, then what is the value of n?

If X̅ = 20 is the mean of 10 observations x1, x2, ... x10; then what is the value of $\displaystyle \sum_{i=1}^{10}\left(\frac{3 x_i-4}{5}\right) ?$ ?

If the mean and the sum of squares of 10 observations are 40 and 16160 respectively, then what is the standard deviation?

Three dice are thrown. What is the probability of getting a sum which is a perfect square?