A laboratory has 22 beakers of liquid in a fume hood, and 8 of the beakers have copper ions in them. Adding three drops of a solution will turn the liquid in a beaker blue if there are copper ions present. If the beakers are tested one by one for copper ions and 45 drops are used before all 8 beakers with copper ions are found, how many beakers without copper ions were tested? Think carefully first, then make a decision:
There were 45 / 3 = 15 beakers tested to find all the beakers with copper ions. Thus, 15 - 8 = 7 beakers without copper ions were tested. So, the answer is 7.