The number of houses sold in a suburb in Sydney and the average monthly mortgage rates for 18 months randomly selected are shown in the following table . Mortgage rate ( % ) Number of houses sold Mortgage rate ( % ) Number of houses sold 7.5 90 9.5 68 9.0 72 6.5 97 7.0 89 8.0 79 10.5 62 9.0 75 10.0 58 10.5 53 9.5 70 9.5 73 8.5 74 11.0 50 10.0 65 7.5 82 11.0 51 8.5 70 a. Draw a scatter diagram with the number of houses sold on the vertical axis . b. Describe the relationship between mortgage rate and number of houses sold .