It is given in the question that
Ann, Chang, and Kareem sent a total of 89 text messages over their cell phones during the weekend. Ann sent 9 fewer messages than Chang. Kareem sent 3 times as many messages as Ann.
Let number of messages Chang sent be x. And Ann sent 9 fewer than Chang, so Ann sent (x-9) messages. And Kareem sent 3 times as many messages as Ann, so Kareem sent 3(x-9) messages.
And the total messages are 89. That is
[tex]x+x-9+3(x-9)=89[/tex]
Combining like terms
[tex]x + x - 9 + 3x-27 =89 \\ 5x -36=89 \\ 5x = 125 \\ x =25[/tex]
So Chang sent 25 messages, Ann sent
[tex]25-9=16 messages[/tex]
And Kareem sent
[tex]3*16 = 48 messages[/tex]