Just looking at your other problem and what I'm guessing is some methods they may want used, I'm going to toss in this alternative.
If you add 5% to something, then you have 105% of the original amount. So if you directly multiply 1600 x 105% = 1600 x 1.05, you'll get directly to the total that was available in inventory.
Then if you sold 85%, you have 15% left over. So you can get directly to that answer by multiplying the 1680 x .15 = amount that remains.
Again, I'm tossing that in because of what I've seen in your other problems. But even so, if you can understand this method, it's actually faster with less calculations.