If the math was translated correctly, then it does indeed seem to be off quite a bit. I'm assuming he actually meant to say the price for making each burger was 1.65 red copper coins, not .65. At a profit of .35 red copper coins, multiplied by 60, would be the same as 3.5 multiplied by 6, or 7 multiplied by 3, so 21 red copper coins.
I am also a little thrown off by him saying "red copper coins" when talking about his burgers, but just "copper coins" when talking about the rental of the stall. So it is possible that "copper coins" are worth more than "red copper coins", but if they get a profit of 1.35 red copper coins per burger, that would be 1 more for each of the 60, so 60+21=81. For 81 "red copper coins" to be more than 20 "copper coins" it would seem to be an exchange rate of 4 "red copper coins" for 1 "copper coin", which seems like an odd point for changing denominations, so I assume that "red copper coins" and "copper coins" are the same thing, and that it was supposed to be 1.65 and .35, but got flipped to .65 and 1.35.
If "red copper coins" and "copper coins" are the same, and his profit margin is actually 1.35 per burger, they would only need to sell 15 in 10 days to make a profit (15x1.35=20.25), which is also seen in the fact that the 60 he said would be 1 more than 4 times the amount he said he needed, so dividing by 4 would make it .25 more than the amount needed.