I went to the doc to get some med before my sore throat worsens. Seemed like my sore throat only got worse after I start teaching for 3 hours non-stop. My mood got worse also and I started to raise my voice and got irritated.

Anyway, maths question for everybody to solve

There are 3 taps which can fill the tank in 20 min. Tap A fills the tank in 45 min. Tap B fills the tank in 60 min. How long does Tap C take to fill the tank?

I gave this qns to an O level student to solve. And he doesn’t know how to. Of course I stressed him by telling him that it is a pri 5 math qns which is true.

So what is the ans?