All together, 3 people have 120 coins. mark has 3 times as many as joel, and joel has 1/2 as many as sandra. how many coins does sandra have?

Joel = j, Mark = m, Sandra = s
j + m + s = 120
m = 3j
j = 1/2 s --> s = 2j
plug in (substitute) each letter to make it in terms of only j: j + m + s = 120
j + 3j + 2j = 120
6j = 120
6j/6 = 120/6
j = 20
now plug in for the j & s equation:
s = 2j = 2(20) = 40
Therefore Sandra has 40 coins!

12x+8-x=12x-x+8=11x+8=11