I want to find a multiple of 30 that is close to 520, but not more than it. I can start by dividing 520 by 30 to see how many times 30 goes into 520. I get 17 with a remainder of 10. This means that 17 times 30 is 510, which is the largest multiple of 30 that is less than 520. I can check this by adding 30 to 510 and seeing that it is more than 520. So the answer is 510. 
What is the largest multiple of 30 which is less than 520?