Median of the data $9,12,1,20,25,5,18,15,21,11$ is $M_1$. If 10 is included in the data, then median of the new data is $M_2$. What is the value of $\left(M_1-M_2\right)$?

1.5

Given Data: 9,12,1,20,25,5,18,15,21,11

Rearranging the data: 1,5,9,11,12,15,18,20,21,25. (Even Count Terms)

Number of terms, n = 10

Median is M1 = average of (n/2)th term and [(n/2)+1]th term = average of 5th term and 6th term = (12+15)/2 = 13.5

Now 10 is included, thus the data is: 1,5,9,10,11,12,15,18,20,21,25 (Odd Count Terms)

Number of terms, n = 11

Median is M2 = [(n+1)/2]th term = 6th term =  12

(M1−M2) = 13.5-12 = 1.5

The correct answer is Option (3) → 1.5