Since the class marks are equally spaced. ∴ Class size = 114 – 104 = 10 If a is a class mark and h is size of class interval, then lower limit and upper limit of the class interval area a - h / 2 and a + h / 2 respectively. ∴ we have h = 10 ∴ Lower limit of first class interval = 104 - 10 / 2 = 99 Upper limit of first class interval = 104 + 10 / 2 = 109 ∴ First class interval is 99 - 109 Hence, the class intervals are 99 - 109, 109 - 119, 119 - 129, 129 - 139, 139 - 149, 149 - 159, 159 - 169.