There can be lots of ways to tweet this, but the main steps are pretty much the same.
Essentially, we are calculating 15 of 0.015, or 15x.0.015. We are doing 0.015 because, with percentage, many times in math problems, we have to convert percentage to a "regular number", moving the decimal point left two places. So, 15 x 0.015 = 0.225. So, that's 0.225 trillion dollars.
From there, it isn't uncommon to change the dollars, given we have a number less than 1. So, when you consider converting trillions to billions, the shortcut is move the decimal place 3 places to the rights. So, 0.225 trillion becomes 225 billion.