Question

One side of a staircase is to be closed in by rectangular planks from the floor to each step. The width of each plank is 9 inches and their height are successively 6 inches, 12 inches, 18 inches and so on. There are 24 planks required in total. Find the area in square feet. 

(a) 112.5 

(b) 107 

(c) 118.5 

(d) 105 

Answer 1

Correct option (a) 112.5    

Explanation: 

The series of plank sizes would be: 0.75 × 0.5, 0.75 × 1, 0.75 × 1.5….0.75 × 12. The sum of this AP is 112.5.