What is the mad of 15, 17, 15, 17, 21, 17, 15, 21, 20, and 18

Okay i will just do this step by step...

1st.) Find the mean of all the values. 
Of course finding the mean is adding all the numbers up together and dividing by the number of numbers present. 
So add all the numbers...
15 + 17 + 15 + 17 + 21 + 17 + 15 + 21 + 20 + 18 = 176
Now divide by 10 since there are ten numbers. 
176 ÷ 10 = 17.6 

2nd.) subtract the mean from each value, (ignore the minus signs in our answer, they are not needed, our answer will be a positive number!!!)
So...
15 - 17.6 = 2.6 
17 - 17.6 = 0.6
15 - 17.6 = 2.6 
17 - 17.6 = 0.6 
21 - 17.6 = 3.4
17 - 17.6 = 0.6 
15 - 17.6 = 2.6 
21 - 17.6 = 3.4
20 - 17.6 = 2.4
18 - 17.6 = 0.4 

Now our new Set of numbers is...
2.6, 0.6, 2.6, 0.6, 3.4, 0.6, 2.6, 3.4, 2.4, 0.4 

3rd.)  So now we have to take those numbers and find the Mean out of those... Soo first add them up together. 
2.6 + 0.6 + 2.6 + 0.6 + 3.4 + 0.6 + 2.6 + 3.4 + 2.4 + 0.4 = 19.2 
Now divide by the number of numbers present or in this case (10)
s
19.2 ÷ 10 = 1.92

s
MAD = 1.92

And if you loose hope, you can always eat a pineapple :)

20000 as first it increases by 2500 then by 5000 so next it will increase by 10000