Question

How many of the first 1000 positive integers are multiples of both 4 and 5 but not 6 ?

Answer 1

Let's begin by breaking  each number down into its prime factors:    4 = 2 x 2    5 = 5    6 = 2 x 3 Next, let's determine the Lowest Common Multiple (LCM) of the numbers 4, 5, and 6 by multiplying all common and unique prime factors of each number:    common prime factors: 2    unique prime factors:  2,5,3    LCM = 2 x 2 x 5 x 3 = 60 Next, let's determine how many times 60 goes into 10,000 (excluding remainder):    10,000/60 = 166 and 2/3    Multiples of ALL 3 numbers (4,5,6) = 166  Next, let's determine the Lowest Common Multiple (LCM) of the numbers 4 and 5 by multiplying all common and unique prime factors of each number:    common prime factors:  none
    unique prime factors:  2 x 2 x 5
    LCM = 2 x 2 x 5 = 20 Next, let's determine how many times 20 goes into 10,000:
    10,000/20 = 500
    Multiples of BOTH numbers (4 and 5) = 500 Finally, let's subtract the multiples of ALL three numbers (4,5,6) from the multiples of BOTH numbers (4 and 5) to get our answer:   Multiples of ONLY numbers 4 and 5 (excluding 6):       500 - 166 = 334