To add on percentages.
Set 100 on C to 100 + given percentage on D, and under original number
on C read result on D.
To deduct percentages.
Set R.H. index of C to 100 − the given percentage on D, and under
original number on C read result on D.
EX.—From £16 deduct 7½ per cent.
Set 10 on C to 92·5 on D and under 16 on C, read 14·8 = £14, 16s. on
D.
To calculate simple interest.
Set 1 on C to rate per cent. on D; bring cursor to period on C and 1 on
C to cursor. Then opposite any sum on C find simple interest on D.
For interest per annum.
Set R.H. index on C to rate on D, and opposite principal on C read
interest on D.
EX.—Find the amount with simple interest of £250 at 8 per cent., and
for a period of 1 year and 9 months.
Set 1 on C to 8 on D; bring cursor to 1·75 on C, and 1 on C to
cursor; then opposite 250 on C read £35, the interest, on D. Then
250 + 35 = £285 = the amount.
To calculate compound interest.
Set the L.H. index of C to the amount of £1 at the given rate of
interest on D, and find the logarithm of this by reading on the reverse
side of the rule, as explained on page 46. Multiply the logarithm, so
found, by the period, and set the result, on the scale of equal parts,
to the index on the under-side of the rule; then opposite any sum on C
read the amount (including compound interest) on D.
EX.—Find the amount of £500 at 5 per cent. for 6 years, with compound
interest.
Set L.H. index of C to £1·05 on D, and read at the index on the
scale of equal parts on the under-side of rule, 0·0212. Multiply by
6, we obtain 0·1272, which, on the scale of equal parts, is placed
to the index in the notch at the end of the rule. Then opposite 500
on C read £670 on D, the amount required, including compound
interest.