An alternative method is to use the pythagorean theorem to find the lengths of BM and MN. Then you can directly compute the area of triangle BMN. You should find that BM = sqrt(20)*x and MN = sqrt(5)*x.
the data set seems wrong, since for every 4twix bars, she had 2 pieces of jolly ranchers; so ratio 4 twix bars : 2 ranchers is to be use but it wouldn't work, it can only work for the first 30 candies ( 20twix bars and 10 ranchers) so the last 2 candies( if following the ratio will be 4/3 for the twix bars and 2/3 for the ranchers)