Through March, Dr. Banks gave out 53 + 67 + 46 = <<53+67+46=166>>166 toothbrushes.
Before giving away toothbrushes in April, he had 330 -166 = <<330-166=164>>164 toothbrushes remaining.
Between April and May, he gave out the remaining 164 / 2 = <<164/2=82>>82 toothbrushes each month.
Dr. Banks gave 82 toothbrushes in his busiest month and 46 toothbrushes during the slowest month.
The difference would be 82- 46 = <<82-46=36>>36 more toothbrushes in the busiest month.
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