If $\rm \displaystyle \sum_{i = 1}^{10}x_i = 110$ and $\rm \displaystyle \sum_{i = 1}^{10}x_i^2 = 1540$  then what is the variance?

3-digit numbers are formed using the digits 1, 3, 7 without repetition of digits. A number is randomly selected. What is the probability that the number is divisible by 3?

What is the probability that the roots of the equation x² + x + n = 0 are real, where n ∈ ℕ and n < 4?

If A and B are two events such that P(not A) = $\rm \frac{7}{10}$, P(not B) = $\rm \frac{3}{10}$ and P(A|B) = $\rm \frac{3}{14}$, then what is P(B|A) equal to?