Why havent we multiplied the number of redirections by 30 to calculate per month?

Traffic estimates: Assuming, we will have 500M new URL shortenings per month, with 100:1 read/write ratio, we can expect 50B redirections during the same period:

100 * 500M => 50B

Is the above calculation / day? Do we have 100: 1 read/write ratio per day? Also, are we saying that 500 M is for 30 days so multiplying that by 100 should give us the total for 30 days itself?

Yes, the given calculation is for 30 days.