The variance of 20 observations is 5. If each observation is multiplied by 3, then what is new variance of the resulting observations?

The mean of a group of 100 observations was found to be 20. Later, it was found that four observations were incorrect, which were recorded as 21, 21, 18 and 20. What is the mean if the incorrect observation are omitted?

The mean weight of 150 students in a certain class is 60 kg. The mean weight of boys in the class is 70 kg and that of girls is 55 kg. What is the number of boys in the class?

If two regression lines between height (x) and weight (y) are 4y $$-$$ 15x + 410 = 0 and 30x $$-$$ 2y $$-$$ 825 = 0, then what will be the correlation coefficient between height and weight?