If it's 20:1, of course you call the raise. On the other hand, if it's a typical online HU game, then it could be like a raise and a call before the flop, a bet and a call on the flop, a bet and a call on the turn, and a bet and raise on the river. Now, that's about 8:1 odds, you have to right more than 11% to be break even. Sometimes I fold my top pair facing an opponent who doesn't raise with top pair on the river.