Question

A red light flashes $3$ times per minute and a green light flashes $5$ times in $2$ minutes at regular intervals. If both lights start flashing at the same time how may times do they flash together in each hour?

• A. $30$
• B. $24$
• C. $20$
• D. $60$

Answer 1

The correct option is A $30$

Red light blinks $3$ times per minutes i.e.
after every $\frac{60}{3}$ seconds $=20$ seconds
Green light blinks $5$ times per $2$ minutes
i.e. after every $\frac{120}{5}$ seconds $=24$ seconds
$\therefore$ Now they will blink after

L.C.M. of $(20$ and $24)$ seconds $=120$ sec $-2$ minutes

Hence, the number of times they blink in
$1$ hour $=\frac{60}{2}=30.$