Mental Ability and Arithmetic - Part 2

Duration: 27 min

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This educational video is a lecture on solving a problem related to finding the number of common terms between two arithmetic progressions (APs). The instructor, Yash Jain Sir from Knowledge Gate Eduventures, begins by presenting a question from the TCS NQT 2020 exam: "Find the number of common terms" between the sequences A) 17, 21, 25, 29, ..., 417 and B) 16, 21, 26, 31, ..., 466. The core of the lesson is a step-by-step algorithm. First, the common difference (d) of each sequence is identified: d1 = 4 for sequence A and d2 = 5 for sequence B. The first common term is found to be 21. The key insight is that the common terms form a new arithmetic progression with a common difference equal to the Least Common Multiple (LCM) of the original two differences, which is LCM(4, 5) = 20. The final step is to use the formula for the nth term of an AP, n = (l - a) / d + 1, where 'l' is the last common term, 'a' is the first common term, and 'd' is the common difference of the new sequence. The instructor demonstrates this by calculating the last common term as 417, then applying the formula: n = (417 - 21) / 20 + 1 = 396 / 20 + 1 = 19.8 + 1, which is not an integer, indicating a need to find the actual last common term. The video also includes a brief, unrelated segment on a different topic, 'THE BADMAAASH BACCHHA', which appears to be a mnemonic or a separate example, but the main focus remains on the AP problem.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video opens with a title slide showing a man writing on a green chalkboard filled with mathematical equations, with the word "ARITHMETIC" at the bottom. This transitions to a black screen with the title "PLAYING WITH AP/GP" in red text. The instructor, Yash Jain Sir, is introduced in a small window, and the video is identified as copyrighted content from Knowledge Gate Eduventures.

  2. 2:00 – 5:00 02:00-05:00

    The main problem is presented on a blackboard: "QUE: FIND THE NUMBER OF COMMON TERMS". The two sequences are given as A) 17 21 25 29 .... 417 and B) 16 21 26 31 .... 466. The instructor states the question was asked in TCS NQT 2020. The video shows the sequences being written out, with the first common term, 21, being highlighted.

  3. 5:00 – 10:00 05:00-10:00

    The instructor begins to solve the problem. He identifies the common difference of sequence A as 4 (21-17) and sequence B as 5 (21-16). He explains that the common terms will form a new AP with a common difference equal to the LCM of 4 and 5, which is 20. He writes the formula for the nth term of an AP: a_n = a + (n-1)d. He then identifies the first common term as 21 and the last common term as 417, which is the last term of sequence A.

  4. 10:00 – 15:00 10:00-15:00

    The instructor writes the formula for the number of terms: n = (l - a) / d + 1. He substitutes the values: l = 417, a = 21, d = 20. He calculates (417 - 21) / 20 + 1 = 396 / 20 + 1 = 19.8 + 1 = 20.8. He notes that this is not an integer, which is a problem. He then realizes that 417 might not be a common term and needs to find the actual last common term that is less than or equal to 417.

  5. 15:00 – 20:00 15:00-20:00

    The instructor corrects the approach. He states that the last common term must be a multiple of 20, starting from 21. He calculates the last common term by finding the largest multiple of 20 that is less than or equal to 417. He finds that 400 is the largest such number. He then applies the formula: n = (400 - 21) / 20 + 1 = 379 / 20 + 1 = 18.95 + 1 = 19.95. He realizes this is still not an integer. He then correctly identifies that the last common term is 401, which is 21 + 19*20. He calculates n = (401 - 21) / 20 + 1 = 380 / 20 + 1 = 19 + 1 = 20. The number of common terms is 20.

  6. 20:00 – 25:00 20:00-25:00

    The video transitions to a different topic, titled "THE BADMAAASH BACCHHA". The instructor draws a circle and writes numbers 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 inside it. He then writes a formula: (a+b)-1, and applies it to the numbers 2 and 3, getting (2+3)-1 = 4. He then writes (5+1)-1 = 5, and 6-1-1 = 4. This appears to be a separate example or a mnemonic for a different concept, possibly related to counting or combinations.

  7. 25:00 – 26:42 25:00-26:42

    The video ends with a final slide that says "THANKS FOR WATCHING" over a blue, abstract background with mathematical symbols. The instructor's video window is still visible in the corner, and the copyright notice for Knowledge Gate Eduventures remains at the bottom.

The video provides a clear, step-by-step solution to a common problem in arithmetic progression. It begins by identifying the key components of the two given sequences: their first terms, common differences, and last terms. The central concept is that the common terms of two APs form a new AP, whose common difference is the LCM of the original two differences. The instructor correctly identifies the first common term and then applies the standard formula for the number of terms in an AP. The video demonstrates a common pitfall: assuming the last term of the original sequence is also a common term, and shows the correct method of finding the actual last common term by checking the sequence of common terms. The final answer is 20. The latter part of the video, on 'THE BADMAAASH BACCHHA', seems to be an unrelated example or a mnemonic, but the core lesson on APs is well-structured and pedagogically sound.