Answer
Please see the work below.
Work Step by Step
We know that
$t=\sqrt{\frac{2h}{g}}$
$t=\sqrt{\frac{2(0.196)}{9.8}}=0.2s$
As there are four drops at 0.2 seconds, the number of drops per second can be determined as
$N=\frac{4}{0.2}=20 drops$