The freezing point of mercury is -39°C. It cannot be used in thermometers for temperatures below this because MCQ with Answer and Explanation

Explanation:
Q = mcΔT = 2×4200×(100-20) = 2×4200×80 = 672000 J = 672 kJ. Direct calorimetry formula application. Memory tip: 'Q = mcΔT; ΔT in Celsius or Kelvin same'. Basic numerical testing specific heat concept, frequently appearing in thermodynamics sections of competitive exams.

Explanation:
Convex lens: object within focal length (u < f) produces virtual, erect, magnified image (magnifying glass). Beyond F: real images. Memory tip: 'Convex lens: u < f ⇒ virtual image; u > f ⇒ real image'. Ray optics concept frequently tested in competitive exams to verify lens image formation rules.

Explanation:
Let distance d, usual time t hours. d/5 = t + 10/60, d/6 = t - 5/60. Subtract: d/5 - d/6 = (15/60) => d/30 = 0.25 => d = 7.5 km. Commonly asked.