A building construction work can be completed by two masons A and B together in 22.5 days. Mason A alone can complete the construction work in 24 days less than mason B alone. Then mason A alone will complete the construction work in:

Solution & Explanation

Core Logic

Let the time taken by mason A alone to complete the work be x days. Mason B takes x + 24 days. Work done by A in 1 day = (1)/(x) Work done by B in 1 day = (1)/(x + 24)

Step 1: Set up the Rate Equation

Since together they finish in 22.5 days, their combined work in 1 day is (1)/(22.5) = (1)/(45/2) = (2)/(45). So, (1)/(x) + (1)/(x + 24) = (2)/(45)

Step 2: Solve the Quadratic

Multiply to clear denominators:

(x + 24 + x)/(x(x + 24)) = (2)/(45) 45(2x + 24) = 2(x² + 24x) 90x + 1080 = 2x² + 48x 2x² - 42x - 1080 = 0 x² - 21x - 540 = 0

Factorizing:

(x - 36)(x + 15) = 0

Since time cannot be negative, we reject x = -15. So, x = 36 days.

Pattern Recognition

Standard "Time & Work" reciprocal addition resolves purely to a clean factorizable quadratic. Setting the faster worker to x prevents dealing with negative bounds in factors.

Chapter Mix

Class 11 Maths: Basic Mathematics

Reference Study Guides

More Basic Mathematics Previous-Year Questions — Page 6

Q20 jee_main_2024_31_jan_morning Sign of Quadratic Expressions
Let S be the set of positive integral values of a for which (ax² + 2(a + 1)x + 9a + 4)/(x² - 8x + 32) < 0, x in R. Then, the number of elements in S is:
  • A. 1
  • B. 0
  • C. ∞
  • D. 3

Solution

Core Logic

For the denominator x² - 8x + 32, D = 64 - 128 < 0 and a = 1 > 0. Thus, x² - 8x + 32 > 0 x in R.

Step 1: Constraint on Numerator

Since the denominator is always positive, the numerator must be strictly negative for all x in R.

ax² + 2(a + 1)x + 9a + 4 < 0 x in R

This requires a < 0 and D < 0.

Step 2: Conclusion

Since a must be strictly less than 0, there are no positive integral values of a that satisfy the condition. Hence, S is an empty set. Number of elements is 0.

Chapter Mix

Class 11 Maths: Quadratic Equations

Rankbit System
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