Probably depends on your aversion to losing it all. If it's a dollar go for it. If it's 100 dollars, again, take the bet. If it's your life savings of 50000 dollars and going to 0 would be worse for you than 70/30 doubling your money, you don't want to go for it.
Sure, much like the Kelly criterion would tell you. If your life savings are $50,000, and someone offers you an even odds bet but with a 70 % chance of winning, you get optimal growth by wagering $20,000. However, that assumes you'll get a large number of similar offers so that you can make your losses back later if you get unlucky now.
