Just go to usps.com and you'll find everything you need there :] pricing and everything
The US postage rate to England for a postcard is $1.10 according the the USPS. This information and other rates may be found on their online postage price calculator.
You only need one!
it cost .26 cents
If this is just a post card you will only need the one stamp to mail it within the US.
TWO US Forever stamps will suffice (USA to London post card)
Just one. Post cards and regular First Class postage is the same for Alaska and Hawaii as for the contiguous states.
usually none, but if it needs a stamp, just one stamp is needed.
3 stamps!
I suppose you need one or two stamps to send the president. But my guess is just one.
4 stamps
You will need a postcard stamp. They currently cost $0.29.
what the price to mail letter to
The number of stamps that you need to send a letter from Australia to London, will depend with the value of one stamp.