Lucy had $72, which is nine times as much money as Xavier had. How much money did Xavier have? Select the correct solution method below, representing Xavier's money with x.
a. Multiply both sides by 9. Xavier had $648.
b.x – 9 = 72. Add 9 to both sides. Xavier had $81.
c.x + 9 = 72. Subtract 9 from both sides. Xavier had $63.
d.9x = 72. Divide both sides by 9. Xavier had $8.