Percentage manhattan


in this if its every 3 days till 30 days so we should take only 10 days why 11 days are taken suppose on day 1 2 cockroaches r there by day 30 there will be 1024 n then 1024-2/2 then 511 can u explain WHY 11 DAYS ARE C

ONSIDERED

But, where does it say 11 days?