Sect. I.

Problem VII.

Three Numbers being given, to find a fourth in an inversed Proportion.

This is called the Rule of Three Inverse, in which observe, that if the third Number be greater than the first, then will the fourth be less than the second. And contrariwise, if the third Number be less than the first, the fourth will be greater than the second; and in either Cases the Rule is this:

    Set the third Number upon A, the first (being of like Denomination) upon B, and then against the second Number upon A, you have the fourth upon B.

    Example: If 8 Men do any piece of Work in 9 Days; In how many Days can 12 Men do the same Work?

    Set 12 upon A, to 8 upon B, then against 9 upon A, is 6 upon B, which is the Answer. For 12 Men may do the same Work in 6 Days, that 8 men will do in 9 Days.

    But if the Question had been, In how many Days can six Men do the same Work? The Answer will be 12: For,

    As 6 upon B, is to 8 upon A, So is 9 upon B, to 12 upon A.

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