The best way to approach something like this is to find the highest common multiple of 12 and 18, which is 6. Therefore, you should find out what the cost of 6 eggs is. In order to find out what 6 cost, if 12 cost $2.09, you simply have to divide this by 2, giving you $1.05 per 6. In order to get 18, you then have to multiply this by three, giving you $3.15 for 18 eggs.
Therefore, if 12 eggs cost $2.05, 18 eggs cost $3.15
Hope this helps :)
