Sara sent letters to her friend in China every month. She sent 6 letters in January, 9 letters in February, and in March she sent triple the number of letters she sent in January. How many letters does Sara send?
Combining January and February, Sara sent 6 + 9 = <T>add(6, 9)=15 letters.
In March, she sent 3 * 6 = <T>multiply(3, 6)=18 letters.
Altogether, Sara sent 15 + 18 = <T>add(15, 18)=33 letters.
