This problem embraces all the preceding examples pertaining
to Interest, and has already been illustrated.
PROBLEM II.
To find the rate per cent., the principal, the interest, and the
time being given.
Ex. 1. A man borrowed $80 for 5 years, and paid ^36 for the
use of it : what was the rate per cent. ?

Analysis. — The interest of $80 at 1 per cent, for 1 year is 80
cents ; (Art. 404 ;) consequently for 5 years it is 5 tirfles as much,
and $.80 X 5=$4. Now since $4 is 1 per cent, on the principal for
the given time, $36 must be -^ of 1 per cent., which is equal to
9 per cent. (Art. 196.)

Or, we may reason thus : Since $4 is 1 per cent, on the princi-
pal for the given time, $36 must be as many per cent, as $4 i»
contained times in $36 ; and $36-^$4=9. Ans. 9 per cent.
QuKST — 420. How many teriQS are connected with eai h of :he preceding eizamples
What arc they ? V^hen three are given, can the fourth be found ? Oht. When li a nam
KT or quantity said tu bo given ?



26Cr APPLICATIONS OF [SeCT. Xll.
Peoof.— |'80X-Cr9=S'7.20, the interest of $80 for 1 year at
9 per cent., and 87.20X5 =$36.00, the interest for 6 years, which
is equal to the sum paid. Hence,

422. To find the raU per cent, when the principal, interest,
and, time are given.
Divide the given, interest by the interest of the principal at 1
jjer cent, for the given time, and the quotient will be the required
per cent.
