Page:Methods of Operating the Comptometer (1895).djvu/34



Multiply the amount of the bill by the negative of the rate of discount. The product will be the net amount of the bill.

Thus: $524.25, 30 per cent off. To strike the negative of 30 we strike the red 2, (see page 11). Therefore, we begin on the second key from the right on the row of red 2's, and strike each red 2 in succession toward the left, as many times as the corresponding figure in the multiplicand indicates. Thus, striking the red 2 in the second column five times, the next red 2 to the left two times, the next four times, the next two times, and the next five times; and pointing off four places, $366.975 appears as the amount of the bill, after discounting.

It is obvious that to find the discount, all that is necessary is to multiply the principal by the rate of discount.

For discounting bills like the following:

Multiply the first item negatively by its per cent. off, and leaving the amount standing on the register, proceed to multiply the other items negatively by their per cent. off. The sum of all the items with their discounts deducted will then be indicated on the register.

Thus: After multiplying $2.16 by 22 negatively, $1.6848 appears on the register, and leaving this amount on the register, and multiplying $7.92 by 30 negatively we have $7.2288, and after multiplying $6.72 by 15 negatively, $12.94 appears as the answer.

$452 less 70%, 47% and 10%.

After multiplying 452 by the red 6 (one less than the 7 of the 70%) we have 135.60. Multiply this by 47 on the red figures (not one less than 47 because 135.60 is already on the machine) we have 71.868, and in the same manner multiply this by red 1 (for the 10%) we have 64.68, the value of the bill after taking off 70%, 47% and 10%.

In multiplying by the red 47 and red 1 use the rule on page 9.

In taking off any per cent. less than 10, remember that such per cent. stated decimally has a cipher before it. Thus, 6% is .06, so for 6% multiply on the row of red naughts for the naught and on the red 5 for the 6.