The first lesson is free.
The lessons 2 to 11 cost 10*$5=$<<10*5=50>>50.
He then had 9 remaining lessons where he had to pay for every other one.
So he pays for lessons 13,15,17&19 = 4 lessons.
Those lessons cost 4*$5=$<<4*5=20>>20.
So in total, the cost was $50+$20=$<<50+20=70>>70.
So he paid $70/2=$<<70/2=35>>35.
#### 35