4. 9.9 The initial cost of a bridge that is expected to have a very long useful life is $70 million. Maintenance can be done at 1-, 2-, 3-, or 4- year intervals, but the longer the interval between servicing, the higher the cost. The cost of servicing is estimated at $83,000, $91,000, $125,000, and $183,000 for intervals of 1 through 4 years, respectively. What interval should be scheduled for maintenance to minimize the overall equivalent annual cost? The interest rate is 8% per year.