If the interest earned by a cd is compounded quarterly, which rule is most accurate when calculation how long it will take the money invested in the cd to double

"jake’s roof repair has provided the following data concerning its costs: fixed cost per month cost per repair-hour wages and salaries $ 20,900 $ 15.00 parts and supplies $ 7.70 equipment depreciation $ 2,800 $ 0.35 truck operating expenses $ 5,720 $ 1.60 rent $ 4,690 administrative expenses $ 3,850 $ 0.50 for example, wages and salaries should be $20,900 plus $15.00 per repair-hour. the company expected to work 2,600 repair-hours in may, but actually worked 2,500 repair-hours. the company expects its sales to be $47.00 per repair-hour. required: compute the company’s activity variances for may."