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!