After Tommy had stacked up 7 chairs in the school field after a celebration, he noticed 3 things. Firstly, each of the chairs was stacked the same height above the other. Secondly, the foot of the fourth chair was 23.4 cm above that of the first. Thirdly, the chairs reached a height of 139 cm. What is the height of each chair?
Number of chairs that contributed to the height of 23.4 cm above the first chair
= 4 - 1
= 3

Height increase with each additional stacked chair
= 23.4 ÷ 3
= 7.8 cm

Number of chairs stacked on top of the first chair
= 7 - 1
= 6

Height increase of 6 additional stacked chairs
= 6 x 7.8
= 46.8 cm

Height of one chair
= 139 - 46.8
= 92.2 cm

Answer(s): 92.2 cm