Rachel decided to have bouncy balls as a party favor. She determines that her ratio of bouncy balls to guests is 9:3. Explain how to find the number of balls needed for 30 guests.

Identify the variables as number of guests, x, to the number of bouncy balls, y. The ratio, 9:3, can be simplified to 3:1 resulting in the constant of variation of 3. The equation y = 3x is used to substitute the 30 guests in for x, y = 3 (30), y = 90, so she needs 90 bouncy balls for 30 guests.

Bouncy balls to guests = 9:3 so every 3 guests you need 9 bouncy balls so if you have 30 guests you simply take

9 x 10 = 90 bouncy balls