LCM between 10 and 19 is 190.
First we can find the prime factorization of 10:
2 * 5.
Prime factorization of 19:
19.
2 * 5 * 19 = 190
The equivalent expression is 
<h3>How to determine the equivalent expression?</h3>
The expression is given as:
rootindex 5 startroot 1,215 endroot superscript x
Rewrite properly as:
![\sqrt[5]{1215^x}](https://tex.z-dn.net/?f=%5Csqrt%5B5%5D%7B1215%5Ex%7D)
Apply the law of indices

Hence, the equivalent expression is 
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If x is the average of "the other 20 numbers", then their sum is 20x. Adding the 21st number makes the sum of all 21 numbers be 20x +4x = 24x.
The 21st number, 4x, is 4x/(24x) = 1/6 of the total of all numbers,