From the Super Mario Wiki
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The player is shown a fly in a jar, and then told to make the number of flies either 7, 9 or 10, by pressing two buttons next to the jar. One of them has "+1" on it, and will add one fly to the jar. The other button has "x2" on it, and will double the amount of flies. The player can press these buttons in any order, as long as it makes the number. For example, if the number of flies to get was 7, the player could press "+1, x2, +1, +1, +1". If the player has made the number of flies by the end, he or she will win. However, if the player does not do this, then they will lose.