I get a black strip every 10 days. A white strip every 20 days, and a red strip every 21 days. I just got all 3 strips. How long 'till I get all of them again on the same day?

What is 7*3*2*5?

210

To determine when you will get all three strips again on the same day, we need to find the least common multiple (LCM) of the three numbers: 10, 20, and 21.

First, let's find the LCM of 10 and 20:
- List the multiples of 10: 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, ...
- List the multiples of 20: 20, 40, 60, 80, 100, 120, 140, 160, ...

From the lists above, we can see that the LCM of 10 and 20 is 20.

Now, let's find the LCM of 20 and 21:
- List the multiples of 20: 20, 40, 60, 80, 100, 120, 140, 160, ...
- List the multiples of 21: 21, 42, 63, 84, 105, 126, 147, ...

From the lists above, we can see that the LCM of 20 and 21 is 420.

Therefore, it will take 420 days for you to get all three strips on the same day again.