To solve this problem, you start with your given information of 15 moles of nitrogen gas. Then you make a molar ratio. You put moles of nitrogen on the bottom and moles of ammonia on the top. You had to balance the reaction by putting the three in front of the hydrogen gas in a two in front of the ammonia. There's a one in front of the nitrogen gas. So the molar ratio is two moles of ammonia to one mole of nitrogen. That means that you multiply 15 times two and you get the answer of 30 moles of ammonia.