Mork and Mindy went to the garden center together to buy plants. Mork bought 6 ferns and 1 azalea for $80. Mindy bought 5 ferns and 3 azaleas for $84. What is the price of a single fern?

Convert the info into equations:

6f + a = 80

5f + 3a = 84

Then substitute one into the other, such that you eliminate one variable:

6f + a = 80 = > a = 80-6f

5f + 3 (80-6f) = 84 = > 5f + 240 - 18f = 84 = > - 13f = - 156 = > f=12

so the fern is $12.