Here's how to break down converting 21.5 trillion to scientific notation:

* Trillion: A trillion is 1,000,000,000,000 (1 followed by 12 zeros).

* 21.5 trillion: This is 21,500,000,000,000

* Scientific Notation: We want to express this as a number between 1 and 10 multiplied by a power of 10. We can move the decimal point 13 places to the left to get 2.15.

Therefore, the exponent needed on base 10 is 13.

So, 21.5 trillion in scientific notation is 2.15 x 10^13.