ANS:B - slightly less than 5% of the design discharge

When the drainage to sewage ratio is 20, it implies that for every 20 units of drainage water, there is 1 unit of sewage. The peak dry weather flow, also known as the maximum dry weather flow (MDWF), typically represents the highest anticipated flow rate in the sewer system during dry weather conditions. Given the drainage to sewage ratio of 20, it means that the sewage component constitutes only 1/21 of the total flow (1 unit of sewage out of 21 units of total flow). Therefore, the peak dry weather flow would be slightly less than 5% of the design discharge since 1/21 is approximately 4.76% of the total flow. So, the correct option is: slightly less than 5% of the design discharge.