A natural number x is chosen at random from the first 100 natural numbers. What is the probability that x² + x > 50?

What is the mean deviation of the first 10 natural numbers?

Let $\rm \Sigma_{i=1}^9x_i^2=885$ If M is the mean and σ is the standard deviation of  x₁, x₂, x₃.....x₉ then what is the value of M² + σ²?

The mean of the series x₁, x₂...x_n is x̅ If x_n is replaced by k, then what is the new mean?