A ladder has rungs 25 cm apart.(see Fig. 5.7). The rungs decreaseuniformly in length from 45 cm at thebottom to 25 cm at the top.

 Number of rungs can be calculated as follows:

2.5 x 100 cm ÷ 25 + 1

= 2.5 x 4 + 1

= 10 + 1 = 11

We have; a = 25, a₁₁ = 45 and n = 11

d can be calculated as follows:

a₁₁ = a + 10d

45 = 25 + 10d

10d = 45 – 25 = 20

d = 2

Total length of wood will be equal to the sum of 11 terms:

                 = 8[ 25 + (10)]

                 = 11 *35

                 = 385 cm