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If somebody says, “a workman receives a pay of ten (50) dirhems per month; how much must be his pay for six days?” Then you know that six days are one-fifth of the month; and that his portion of the dirhems must be proportionate to the portion of the month. You calculate it by observing that one month, or thirty days, is the measure, ten dirhems the price, six days the quantity, and his portion the sum. Multiply the price, that is, ten, by the quantity, which is proportionate to it, namely, six; the product is sixty. Divide this by thirty, which is the known number of the measure. The quotient is two dirhems, and this is the sum. This is the proceeding by which all transactions concerning exchange or measures or weights are settled.
MENSURATION.
Know that the meaning of the expression “one by one” is mensuration: one yard (in length) by one yard (in breadth) being understood.
Every quadrangle of equal sides and angles, which has one yard for every side, has also one for its area. Has such a quadrangle two yards for its side, then the area of the quadrangle is four times the area of a quadrangle, the side of which is one yard. The same takes place with three by three, and so on, ascending or descending: for instance, a half by a half, which gives
