So first of all you need to find out how many even numbers there are from 1-900 (which is 450) so you know that 450 are open. In the 3 multiplication tables every second number is even so you know that half of the 450 lockers that was opened was closed again: this meant that 225 lockers remained open.
You also know that every number in the 4 multiplication tables is the second number in the 2 multiplication tables so half of them are closed but you also know that the 900th locker was opened so now you have 113.
So to conclude you do 900-113 which gives you 797 (this is because 113 is the amount of lockers that is open)
