I need help, please

Q 3. The managers of an import agency are investigating the length of time that customers take to pay their invoices, the normal terms for which are 30 days net. They have checked the payment record of 100 customers chosen at random and have compiled the following table: /10Marks Payment in Number of customers 5 to 9 days 4 10 to 14 days 10 15 to 19 days 17 20 to 24 days 20 25 to 29 days 22 30 to 34 days 16 35 to 39 days 8 40 to 44 days Required: a) Calculate the arithmetic mean. b) Calculate the standard deviation c) Construct a histogram and insert the modal value. d) Estimate the probability that an unpaid invoice chosen at random will be between 30 and 39 days old