Find the missing terms a17 , a19 and a22 of the sequence,
15 , 20 , 13 , 2 , 20 , 17 , 17 , 19 , 19 , 15 , 14 , 4 , 21 , 4 , 20 , 17 , a17 , 19 , a19 , 15 , 14 , a22
The sequence uses only basic Mathematics and less common English.
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The values are
a17 = 17, a19 = 19, a22 = 22
The sequence might be easier to see if we first
Use A1Z26 to convert all elements to letters: o , t , m , b , t , q , q , s , s , o , n , d , u , d , t , q , ? , s , ? , o , n , ?
Which can then be seen to mean:
The powers of one thousand: one, thousand, million, billion, trillion, quadrillion, quintillion, ...
The missing elements are Quinquadecillion, Septendecillion and Vigintillion, which converted back to numbers are 17, 19 and 22 respectively.