[Math] What time are the minute and hour hands of a clock perpendicular

algebra-precalculusangle

At noon the minute and hour hands of a clock coincide. Assuming the hands of the clock move continuously:
a) What is the first time $T_1$ when they are perpendicular?
b) What is the next time $T_2$ when they again coincide?

I believe I have solved this problem, but I would like someone to verify my answer.

I got $T_1$ to be 12:16 P.M. and 21.818181… seconds ($\frac{180}{11}$ minutes after noon).

I got $T_2$ to be 1:05 P.M. and 27.272727… seconds ($\frac{720}{11}$ minutes after noon).

If there are any questions relating to this problem, please feel free to ask them, as I will do my best to answer them.

Best Answer

If the minute hand has made $x$ revolutions around the clock ($60x$ minutes), the hour hand has made $\frac1{12}x$ revolutions; the difference between these two values represents an angle. Both hands start at 0 revolutions.

For (1), the relevant angle is $\frac14$ of the circle: $$x=\frac14+\frac1{12}x$$ Solving, we get $x=\frac3{11}$, which corresponds to a time of $\frac{180}{11}$ minutes after noon.

For (2) we replace $\frac14$ with 1, since the minute hand has lapped the hour hand. Then $$x=1+\frac1{12}x$$ Solving, we get $x=\frac{12}{11}$, i.e. $\frac{720}{11}$ minutes after noon.

Therefore, both your solutions are correct.