 Hi and welcome to the session. I am Deepika here. Let's discuss the question. A ladder has rungs 25 centimeter apart see figure The rungs decrease uniformly in length from 45 centimeter at the bottom to 25 centimeter at the top If the top and the bottom rungs are two and a half meter apart What is the length of the wood required for the rungs? Hinked number of rungs is equal to 250 upon 25. So let's start the solution We know that Some of first and comes off an AP as soon as equal to n by 2 a plus l Where a is the first term l is the last term So even in an AP if first and the last term is given the sum of its first and comes is given by as Then is equal to n by 2 a plus l. So this is the key idea behind this question We will use this key idea to solve the above question So let's start the solution. It is given that the gap between two consecutive rungs is 25 centimeter and the top and Bottom rungs are two and a half meter apart. So the gap gap between two rungs is is equal to 25 centimeter gap between top and Bottom rung is equal to 2.5 meter two and a half meter Which is equal to 2.5 meter? This is again equal to 250 centimeter therefore Number of rungs is equal to 250 upon 25, which is equal to 10 now it is given that the rungs are decreasing uniformly in length from 45 centimeter at the bottom to 25 centimeter at the top therefore according to the question the length of Rungs form an AP Because they are decreasing uniformly as the first round with a is equal to 45 centimeter and Which is the last round because our number of rungs is 10. So l which is the last round is 10 is equal to 25 centimeter would require for the rungs is equal to 45 centimeter is equal to 25 centimeter So according to the key idea we have as soon as equal to n by 2 into a plus l Into a is 45 plus 25 is equal to this is equal to 25 centimeter Therefore the length of the wood required for the rungs is 350 centimeter So for the book question is that is the length of the wood required for the rungs is 350 centimeter. I hope the question is clear to you. Bye and have a nice day