A person starts multiplying consecutive positive integers from 20. How many numbers should he multiply before that he will have result that will end with 3 zeroes?

To get a number that ends with three zeros, we need to multiply numbers that have 2, 5, and 10 as factors because 101010 = 1000, which ends with three zeros.

Step 1: Find the numbers with 2 as a factor. Every even number is a multiple of 2. So, 20, 22, 24, 26, 28, etc., are multiples of 2.

Step 2: Find the numbers with 5 as a factor. Every fifth number is a multiple of 5. So, 20, 25, 30, 35, etc., are multiples of 5.

Step 3: Find the numbers with 10 as a factor. Every tenth number is a multiple of 10. So, 20, 30, 40, 50, etc., are multiples of 10.

To get a number that ends with three zeros, we need to multiply at least three numbers that are multiples of 10. So, the person should multiply at least 30 numbers starting from 20 to get a result that ends with three zeros. This is because the 30th number, 50, is the third number that is a multiple of 10.