Not too sure about the above answer...
In any event. Breakeven is equal to: Fixed Costs/Selling Price less your variable costs.
In this scenario fixed costs are the $800 per month. Selling price is the $60 (which is not the variable cost per your question) rather the $20 would be considered the variable cost. Bear in mind if it were the adverse there would never be a breakeven as it could theoretically not happen.
So, 800/60-20 or 800/40 or 20 lessons.
To check; 20 lessons at 60 equals 1,200
20 lessons at 20 equals (400) * this is in brackets indicating a negative integer as this is the VC.
(800) * Fixed costs