Percent Problems: Practice

Simple interest is $I = P r t$ with $P = 800$, $r = 0.05$, and $t = 3$.

$I = 800 \times 0.05 \times 3 = 120$

The $920$ would be the total balance, not the interest alone.

Multiply the factors rather than adding the rates.

$1.10 \times 0.90 = 0.99$

The stock is at $99\%$ of the start, a $1\%$ decrease.

The interest is $I = P r t = 1200 \times 0.04 \times 2 = 96$, and the total is $P + I$.

$1200 + 96 = 1296$

A tip multiplies the bill by $1 + 0.08 = 1.08$, so divide to reverse it.

$1.08\,w = 54 \implies w = \frac{54}{1.08} = 50$

From $I = P r t$, solve for the rate $r = \frac{I}{P t}$.

$r = \frac{75}{500 \times 3} = \frac{75}{1500} = 0.05 = 5\%$

A $40\%$ discount leaves $60\%$, so $90$ is $0.60$ of the original. Divide to reverse.

$0.60\,w = 90 \implies w = \frac{90}{0.60} = 150$

Multiply by $0.90$ and then $0.80$, the two discount factors.

$250 \times 0.90 \times 0.80 = 250 \times 0.72 = 180$

The combined discount is $28\%$, not $30\%$.

From $I = P r t$, solve for the time $t = \frac{I}{P r}$.

$t = \frac{480}{2000 \times 0.06} = \frac{480}{120} = 4$

First $75\%$ of $240$ is $0.75 \times 240 = 180$. The number times $1.25$ equals $180$, so divide.

$1.25\,n = 180 \implies n = \frac{180}{1.25} = 144$

A markup multiplies the cost by $1 + 0.25 = 1.25$, so divide the price by that factor.

$1.25\,c = 240 \implies c = \frac{240}{1.25} = 192$

The interest is $I = P r t = 1500 \times 0.04 \times 5 = 300$, and the balance is $P + I$.

$1500 + 300 = 1800$

Multiply by $1.10$ twice, once for each year's growth.

$2000 \times 1.10 \times 1.10 = 2000 \times 1.21 = 2420$

Two $10\%$ increases give $21\%$, not $20\%$, because of the change on the change.