X.

$1$ plus the product of the rate and time.

149. Given the Amount, Principal, and Rate. Find the Time.

lint From formula 1, p + prt = a. lint Transpose $p$, prt = a - p. lint Divide by $pr$, t = a - ppr. rint(Formula 4.)

Subtract the principal from the amount, and divide the result by the product of the principal and rate.

150. Given the Amount, Principal, and Time. Find the Rate.

lint From formula 1, p + prt = a. lint Transpose $p$, prt = a - p. lint Divide by $pt$, r = a - ppt. rint(Formula 5.)

Subtract the principal from the amount, and divide the result by the product of the principal and time.

Exercise 64.

Solve the following examples by the preceding formulas:

1. The sum of two angles is $120°{30}^{\prime }{30}^{″}$ and their difference $59°{30}^{\prime }{30}^{″}$. Find the angles.

2. Find the interest of $$1000$ for $3$ years and $4$ months at $4$%.

3. Find the principal that will amount to $$2280$ in $3$ years and $6$ months at $4$%.