What is 15 is 30 of what number?

The phrase "15 is 30 of what number" can be translated to the equation:

15 = 30 * x

Where x represents the unknown number that is being asked for. To solve for x, divide both sides of the equation by 30:

x = 15 / 30 x = 0.5

Therefore, the unknown number is 0.5.