Help with this gcse chemistry question?

To work out the percentage yield you have to do the (actual amount in moles divided by the theoretical amount in moles) * 100. To work out the actual amount in moles of NH3 we need to do mass / mr, so it is 1.8/17 which gives us 9/85mol. To find the theoretical amount in moles we need to find the moles of N2 and then equate this to 2NH3. So, moles of N2 = 7/28 which gives us 0.25mol and as it is a 1:2 ratio amount of moles of NH3 is 0.5mol. Now we put these values into our equation: ((9/85)/0.5)*100 which gives us 21.1764.....% which is 21.2% to 3 significant figures.