"Hot Air Balloon" joke

Three men are in a hot air balloon. Soon, they find themselves lost in a canyon
somewhere.
One of the three men says, "I have got an idea. We can call for help in this
canyon and the echo will carry our voices far." He leans over the basket and
yells out, "Helllloooooo! Where are we?"
They hear the echo several times. 15 minutes later, they hear this echoing
voice, "Helllloooooo! You are lost!"
One of the men says, "That must have been a mathematician."
Puzzled, one of the other men asks, "Why do you say that?"
The man replies, "For three reasons: 1) he took a long time to answer, 2) he was
absolutely correct, and 3) his answer was absolutely useless."