An oblique way of saying "Do you want to bet on it?" -- bet me a fiver and if you're right, you'll win a ten-spot. Uncertain origin.

"Oh that cement is just, it's there for weight, dear. Five will get you ten old MacHeath's back in town."--in Mack the Knife from The Threepenny Opera

The expression 'five will get you ten' indicates that the speaker is comfortably sure that what follows in the statement is true. It comes from the expression of probability as 'odds' in traditional gambling parlance. The first number, five refers to the bet layer's bet amount. The second number, ten, does not refer to an actual bet; it refers to the taker's bet amount that would 'even the odds', which is to say that betting that amount would make the probability of winning and losing the same over a sufficient number of bets. At even odds, both betters have an equal chance of winning or losing.

The expression is generally normalized by dividing both the layer's bet and the taker's bet by the amount of the lower bet. Thus, 'five'll get you ten' becomes the odds of 1 to 2. That's equivalent to the probability of the bet layer's winning one out of three trials (33.3%) and the taker's winning two out of three trials (66.7%). That's rather good odds for the bet taker.

Huh?

For example, if I lay the stake of ten dollars and the natural odds are 2 to 1 (in my favor), then your stake of five dollars against mine would mean that over many trials neither of us would win or lose money. Your lower bet reflects the risk of your higher probability of losing, and compensates for it by winning bigger when you do win and losing less when you lose. The 'getting' in 'five will get you ten' is not the money the bet layer will take in from a particular bet, and because this bet evens the odds (50% to win and 50% to lose) it is not a good bet to make. No sensible gambler will take 50-50 bets ('even odds') as a rule, as there is no favor to the gambler in the outcome. An actual good bet would be less than five against my ten. In fair games of chance, like poker, players try to modify the original even odds to their favor by skillful playing. Still, the great profits of casinos show that masses of people are willing to knowlingly and very persistently bet 'against odds'. It is important to remember that probabilities do not accurately predict outcomes of individual plays (events); they do accurately predict total outcomes for large numbers of plays, and the larger the number of plays, the more accurate the predictions.

In summary, the colloquial meaning of 'five'll get you ten that MacHeath's back in town' is that it's very likely (probable) that MacHeath is back.

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