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As we seem to have veered to include numerical problems on the site,here is one for all you motorists(and others).

There is a hill one mile up and one mile down the other side.

A car drives up the hill at a speed of 30mph. At what speed does it have to travel down hill to average 60mph over the whole journey?

90 mph? the fact it is a hill is irrelevant, isn't it?

i'm with wordsmith with the answer

Sorry Wordsmith and Helena. Not the correct answer.
TJ

He has to travel 2 miles in two minutes to average 60mph over the whole distance. He has already taken 2 minutes to get to the top at 30mph therefore it is impossible for him to get there in time without travelling at infinite speed.
Have I got ths completely wrong?

I think it is not possible.

same reasoning as Harrhy

Harrhy and trevor are right. Harrhy's explanation or should I say proof is spot on.

doh, a bit technical for me

That's a relief! Average speeds are always more complicated than they look:-)