What do you mean the store made more profit at $350? At $350, the store made $5,000 profit [$100*(350-300)] while if the price had been left at $400 immediately prior to the spike in sales they could have realized a $10,000 profit [$100*(400-300)]. >:*3
We are comparing the 20 refrigerators at $400 (20 x$100)with the 100 refrigerators at $350 (100x$50). The extra 80 refrigerators were probably sold because of the attractive lower price! It's called "market elasticity".