Quant Boosters by Shubham Ranka, IIM Calcutta



  • 2x^2 -5x + 3 = t
    2x^2 -5x + 3 - t = 0
    x = 5 +- rt(1+8t))/4
    == > f(t) = 3[5 +- rt(1+8t))/4] + 2 = [23 +- 3rt(1+8t)]/4
    x^2 = 1/16[26+8t+-10rt(1+8t)]
    == > f(x^2) = f(1/8[13+4t+-5rt(1+8t)]) = [23 +- 3rt(p)]/4
    where, p = 1+8*1/8[13+4t+-5rt(1+8t)] = 14+4t+-5rt(1+8t)
    Converting to x :
    1+8t = 16x^2 -40x + 25 = (4x-5)^2
    == > p = 14+4(2x^2 -5x + 3) +- 5(4x-5) = 8x^2 -20x +26 +- (20x-25) = 8x^2 +1 or 8x^2 -40x +51
    f(x^2) = 23+-3rt(8x^2 +1 or 8x^2 -40x +51) / 4
    Since, f(0) = 5
    f(x^2) = [23 - 3rt(8x^2+1)]/4 satisfies



  • Q5) How many times in a day, the hands of a clock are straight?
    a. 22
    b. 24
    c. 44
    d. 48



  • Straight == > 0 degree or 180 degree.
    It makes an angle of 180 degrees 11 times in one circle. 00:30:xx, 1:35:xx, 4:50:xx, 6:00, 7:05:xx, ... 11:25:xx
    == > 22 times.

    0 degrees -- > 00:00: 1:05, ..10:50:xx. == > 2*11 = 22 times

    == > 22 + 22 = 44 times (C)



  • Q6) The last day of a century cannot be
    a. Monday
    b. Wednesday
    c. Tuesday
    d. Friday



  • Calendar cycle repeats in 400 years.
    100 years -- > 5 odd days -- > Friday
    200 years -- > 3 odd days -- > Wed
    300 odd days -- > 1 odd day -- > Mon
    400 odd days -- > 0 odd day -- > Sunday
    == > (C)



  • Q7) The shopkeeper charged 12 rupees for a bunch of chocolate. but I bargained to shopkeeper and got two extra ones, and that made them cost one rupee for dozen less than first asking price. How many chocolates I received in 12 rupees?



  • n chocolates = Rs 12 à price per dozen = 144/n
    I got n+2 in rs 12 = > Price per dozen = 144/n+2
    144/n+2 = 144/n - 1
    == > n = 16



  • Q8) Determine the positive integer values for n for which n^2 + 2 divides 2 + 2001n



  • (n^2 + 2) < = 2 + 2001n == > n < = 2001
    ^2 + 2 divides 2 + 2001n == > It will also divide 2+2001n - (n^2+2) = 2001n-n^2
    n^2 + 2 divides 2001n-n^2 ----(1)

    Case-1:
    If n is even = 2k:
    4k^2 + 2 divides (4002k - 4k^2)
    2k^2 + 1 divides 2001k-2k^2 = k(2001-2k)
    == > 2k^2 + 1 divided 2001-2k -(A)
    Again, Since 2k^2 + 1 divides 2001k-2k^2, it will also divide 2001k-2k^2 + (2k^2 +1)= 2001k+1
    === > 2k^1 + 1 divides (2001-2k) and (2001k+1)
    Trying to remove k:
    2k^1 + 1 will also divides 2001(2001-2k) + 2(2001k+1) = 2001^2 + 2 = 4004003 = 19832539
    By hit and trial, k = 0,3 == > n = 6 (positive integer)

    Case-2:
    If n is odd:
    n^2 + 2 dvides 2001n-n^2 = n(2001-n)
    == > n^2 + 2 divides 2001-n -------(1)
    Also, since n^2 + 2 dvides 2001n-n^2, it also divides: 2001n-n^2 + (n^2+2) = 2001n+2 --(2)
    Trying to remove n:
    2001*(1) + (2):
    n^2 + 2 divides 2001^2 + 2 = 19832539
    Using hit and trial:
    == > n = 9, 2001
    Hence n = 6,9,2001



  • Q9) Ram and Kanti working together can complete a job in 11(13/16) days. Ram started working alone and quit the job after completing 1/3rd of it, and then Kanti completed the remaining work. They took 25 days to complete the work in this manner. How many days could Kanti have taken to complete the job working alone?



  • Let ram takes 3x no. of days to complete the task
    And Kanti takes 3y no. of days to complete the task
    == > 3xy/x+y = 11(13/16) = 189/16
    == > 16xy = 63x+63y ---(1)
    == > xy should be a multiple of 63 = 7*9 == > either of x or y should be a multiple of 7
    1/3rd work -- Ram --x days
    2/3rd work--Kanti --2y days
    == > x+2y = 25 ----(2)
    Now, either of x or y should be 7/21 == > x = odd == > 7
    x = 7 == > y =9 == > satisfies 1
    Kranti can complete the work in 3y = 27 days, working alone.



  • Q10) If the sum of two distinct natural numbers is 50, then what can be the maximum possible HCF of these 2 numbers ?
    (a) 5
    (b) 10
    (c) 11
    (d) 12



  • k (a+b) = 50
    a and b are distinct and a,b > = 1 and a+b > =3 == > k < =17
    k will be a factor of 50 < =27 == > 10,5,1
    Max. of k = 10



  • Q11) In how many ways can 6 letters A, B, C, D, E and F be arranged in a row such that D is always somewhere between A and B?



  • 6! - 2 * x
    x = D _ _ _ _ _ = 5! = 120
    = _ D _ _ _ _ = 3c1 (Sice A/B can not come in the first place now) * 4! = 72
    = _ _ D _ _ _ = 3c2 * 3! * 2! = 36
    = _ _ _ D _ _ = 3! * 2! = 12
    == > x = 120 + 72 + 36 + 12 = 240
    == > Total no. of ways = 720 = 480 = 240



  • Q 12) 2 years ago, fathers age was 6 times of his son's age. 6 years hence the ratio between the ages of father and son is 10:3. What is fathers present age?
    a) 40
    b) 42
    c) 44
    d) 48



  • 2 years ago, let ages be: F S
    F = 6s
    F+8/S+8 = 10/3
    3(6S+8 ) = 10(S+8 )
    S = 7
    F = 42
    == > Current age is 44



  • Q 13) A square is divided into 108 identical rectangles. The ratio of length to breadth of the rectangle is 3:4. If the diagonal of the rectangle measures 1 cm, then the length of the diagonal of the square is closest to
    a) 9 cm
    b) 10 cm
    c) 11 cm
    d) 12 cm



  • Let the side of square be a, and rectangle be 3y, 4y
    == > a^2 = 108 * 12y^2 --- (1)
    Length of diagonal, 5y=1 == > y=1/5
    == > length of diagonal of square = rt(2)a = rt(2 * 108 * 12) * 1/5 = 10.18 --- > (B )



  • Q 14) If 3, 2 and 2 are three altitudes of a triangle then find the area & perimeter of the triangle.


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