G8.1 From 1 to 121 , there are $a$ numbers which are multiplies of 3 or 5 . Find $a$.
Number of multiples of $3=40 \quad(120=3 \times 40)$
Number of multiples of $5=24 \quad(120=5 \times 24)$
Number of multiples of $15=8 \quad(120=15 \times 8)$
Number of multiples of 3 or $5=a=40+24-8=56$