David's optometrist sold him a bottle of eyeglass cleaner containing 30 ml of glasscleaning solution. assuming there are 20 drops per milliliter, and assuming proper cleaning requires 3 drops of glass cleaner on each side of each lens, what is the maximum number of times david can properly clean his glasses before he must buy a new bottle of eyeglas

Each complete cleaning requires 6 drops of cleaner.  Let's convert that to ml:

6 drops     1 ml              3 ml cleaner
* = = 0.3 ml cleaner per cleaning
                 20 drops      10 cleanings

Recall that 1 bottle contains 30 ml cleaning fluid.

How many cleanings is one bottle of fluid good for?

1 bottle       0.3 ml
=   => x = 100 cleanings/bottle
      x            30 ml