At a candy store Erica bought 3 kg of cinnamon red hots and 1 kg of gummy bears for $21 meanwhile Irene bought 3 kg of cinnamon red hots and 3 kg of gummy bears for $39 what is the cost of 1 kg of each type of candy?

You use simultaneous to work this out.
Say c= cinnamon red hots 
       g= gummbears

3c + 3g = 39
3c + 1g = 21
Since the c's are the same you subtract everything. 
So 3c -3c, 3g-1g,  39-21

2g = 18 (You divide by 2 to get what g is)
g = (So 1kg of gummybears = $9)

To get cinnamon red hots, subsitute the g =9 into either the first or second equation at the top:

3c + 1g = 21
3c + 9 = 21 (we know that g=9)
3c = 12 (Subtractt 9 from 21)
c = 4 (divide both sides by 3 to get what c is)
So 1kg of cinnamon red hots = $4