The setup
Volume (mL) × Drop factor (gtt/mL) ÷ Time (min) = gtt/min
Practice uses the common drop factors 10, 12, 15, 20, and 60 gtt/mL. Convert hours to minutes first (× 60).
Rounding
You cannot count part of a drop, so each question says “Round the final answer to the nearest whole drop.” Round only at the end.
250 mL × 20 ÷ 120 min = 41.666… → 42 gtt/min