Question

An electric kettle has two heating coils. When one coil is used, water in the kettle boils in 5 minutes, while when second coil is used, same water boils in 10 minutes. If the two coils, connected in parallel are used simultaneously, the same water will boil in time

• A. 3 min 20 sec
• B. 5 min
• C. 7 min 30 sec
• D. 2 min 30 sec

Answer 1

Answer: (A)

3 min 20 sec

Answer 2

In parallel $\frac{1}{t_{p}} = \frac{1}{t_{1}} + \frac{1}{t_{2}}$ ⇒ $t_{p} = \frac{t_{1}t_{2}}{t_{1} + t_{2}}$

$= \frac{5 \times 10}{5 + 10} = \frac{50}{15} = 3.33\min = 3min.20sec$