If the bowl weighs 520 grams when it holds 15 tablespoons of sugar and 680 grams when it holds 26 tablespoons of sugar, what is the rate of change of the weight of the bowl with regards to the tablespoons of sugar?

The management of a supermarket wants to adopt a new promotional policy of giving a free gift to every customer who spends more than a certain amount per visit at this supermarket. the expectation of the management is that after this promotional policy is advertised, the expenditures for all customers at this supermarket will be normally distributed with a mean of $95 and a standard deviation of $20. if the management wants to give free gifts to at most 10% of the customers, what should the amount be above which a customer would receive a free gift?