To find the smallest number that when added to 10000 becomes divisible by 20, 24, and 30, we need to find the least common multiple (LCM) of these three numbers.

First, let`s find the LCM of 20, 24, and 30:

- The prime factors of 20 are 2² * 5.

- The prime factors of 24 are 2³ * 3.

- The prime factors of 30 are 2 * 3 * 5.

To find the LCM, we take the highest power of each prime factor:

- The LCM of 20, 24, and 30 is 2³ * 3 * 5 = 120.

Now, we need to find the difference between 10000 and the nearest multiple of 120:

- The nearest multiple of 120 to 10000 is 9996 (120 * 83).

- So, the smallest number needed to be added to 10000 is 83.

Therefore, option 3, 80, is the correct answer.