Weekly sales with the El Barato Supermarket Family Card are on average $2,570 in its Mayagez store with a standard deviation of $680. Determine what

Weekly sales with the El Barato Supermarket Family Card are on average $2,570 in its Mayagüez store with a standard deviation of $680. 

Determine what is the probability that in a sample of 81 weeks 

a) the sample average weekly sales is at least $2,300? 

b) Is the sample average weekly sales between $2,400 and $2,600?