It would have been a lot easier for him if he had simply sold the bananas at a flat per-banana price. That's typically how many grocery stores in the US sell them (notably Trader Joe's). Here in Japan, they typically sell them in small bunches for a fixed price, with the small bananas sold at a lower price (for a group of 3 or 4) and the larger ones at a higher price next to them. There's no need to weigh bananas at the point at the point of sale at all.