All that you are being asked to do in this question is to figure out a percentage. The size of the US economy does not matter here. It is also not really important for you to even know what gross domestic product (GDP) is. All you are being asked is to determine how much larger the economy would be if it increased by 1.5%.
So, what you have to do here is to figure out what 1.5% of 15 trillion is. To find this, you must first express 1.5% as a decimal. You move the decimal point two places to the left and you have .015. Now, you multiply .015 by 15. The product of that operation is .225. That means that the US economy will produce $.225 trillion more than it did before. This can also be expressed as $225 billion.
So, if the US economy starts out at $15 trillion and increases by 1.5%, its output increases by $225 billion.
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.
This is a question of figuring out the percentage, economy or GDP has nothing to do with it.
We need to figure out what 1.5% of 15 trillion is.
We can either calculate it using percentage or fraction.
15 trillion * 1.5%= 15 trillion * (1.5/100)
= 0.225 trillion or 225 billion.
As fraction: 15% can also be written as 0.15.
Thus, 15 trillion * 0.15 = 0.225 trillion or 225 billion.
Therefore 1.5% of $15 trillion is $225 billion. In other words, if the current size of US economy is $15 trillion and the GDP increases by 1.5%, the output increases by $225 billion.
Hope this helps.