Imagine Susan, Rueben and Josh in a equation.
x = Susan
y = Rueben
z = Josh
x + y + z = 89 (they wrote 89 messages together)
z = 3x (Josh sent 3 times as many as Susan)
x = y + 6 (Susan sent 6 more than Rueben)
we can turn the x = y + 6 around y = x - 6
now we can add y = x - 6 and z = 3x into the first equation
x + x - 6 + 3x = 89
5x = 95
x = 19
y = 19 - 6
y = 13
z = 3*19
z = 57
Susan sent 19 messages, Rueben sent 13 messages, and Josh sent 7 messages :)
