The management of a supermarket wants to adopt a new promotional policy of giving free gifts to every customer who spends more than a certain amount per visit at this supermarket. The expectation of...

The management of a supermarket wants to adopt a new promotional policy of giving free gifts 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 expenditure for all customers at this supermarket will be normally distributed with a mean 400 £ and a variance of 900 £2.

What is the probability that the expenditure of the first customer exceeds the expenditure of the second customer by at least 20 £?