You should know that the scientific nomenclature that is regularly used to express a trillion is:
1X10^9
<u>
But since the base that accompanies base 10 must always be a number less than 10, you cannot use 21.5 (which would be the ideal) but it must be the number with the decimal sign moved once to the left: 2.15</u> (the which is less than 10), since the decimal sign has been moved to the left in the base, you must increase the exponent by 1 for each digit that you ran the decimal sign, <u>as in this case the sign moved only one digit, you must increase by one the exponent, therefore the exponent will no longer be 9 but 10</u>.