Question

a flashlight can shine brightly for 10 hours or dimly for 100 hours. if it shines brightly for 3 hours and shines dimly for 40 hours, how many more hours can it shine dimly?
a 24 hours
25 hours
c 28 hours
d 30 hours

Explanation:

Step1: Calculate used battery (bright)

Let total battery capacity = 1 unit. Bright rate: $\frac{1}{10}$ per hour. Used: $3 \times \frac{1}{10} = \frac{3}{10}$

Step2: Calculate used battery (dim)

Dim rate: $\frac{1}{100}$ per hour. Used: $40 \times \frac{1}{100} = \frac{40}{100} = \frac{2}{5}$

Step3: Find remaining battery

Remaining = $1 - \frac{3}{10} - \frac{2}{5} = 1 - \frac{3}{10} - \frac{4}{10} = \frac{3}{10}$

Step4: Calculate remaining dim hours

Hours = $\frac{\text{Remaining Battery}}{\text{Dim Rate}} = \frac{\frac{3}{10}}{\frac{1}{100}} = \frac{3}{10} \times 100 = 30$

Answer:

D 30 hours