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!
