The greatest 4-digit number exactly divisible by 10, 15, 20 is = ?

Answer: Option B

LCM of (10, 15, 20)
⇒ 5 × 2 × 3 × 2 = 60
⇒ Largest 4-digit number = 9999
divide 9999 by LCM of given number
⇒ We get remainder = 39
⇒ So, to divide completely subtract it from (9999 - 39) = 9960
∴ 9960 is the largest four digit number which is completely divided by the given numbers.