Question

The speed of a car is $80km/hour$. It is

a) increased by $10\%$.

b) decreased by $10\%$.

Find the new speed of the car in the two cases.

Answer 1

Explanation for a):

Step 1: Finding the increase in speed equivalent to $10\%$.

Speed of car $=80km/hour$

Increase in speed is of $10\%$, which will be equivalent to,

$$\begin{gathered} \Rightarrow 10\%of80km/hr \\ \Rightarrow \frac{10}{100}\times 80=\frac{1\cancel{0}\times 8\cancel{0}}{1\cancel{0}\cancel{0}}=8km/hr \end{gathered}$$

Thus, the speed will be increased by $8km/hour$.

Step 2: Finding the new speed.

$\Rightarrow$ Original speed $+$Increase in speed

$\Rightarrow 80km/hr+8km/hr=88km/hr$

Hence, the new speed will be $88km/hour$.

Explanation for b):

Step 1: Finding the decrease in speed equivalent to $10\%$.

Speed of car $=80km/hour$

Decrease in speed is of $10\%$, which will be equivalent to,

$$\begin{gathered} \Rightarrow 10\%of80km/hr \\ \Rightarrow \frac{10}{100}\times 80=\frac{1\cancel{0}\times 8\cancel{0}}{1\cancel{0}\cancel{0}}=8km/hr \end{gathered}$$

Thus, the speed will be decreased by $8km/hour$.

Step 2: Finding the new speed.

$\Rightarrow$ Original speed $-$decrease in speed

$\Rightarrow 80km/hr-8km/hr=72km/hr$

Hence, the new speed will be $72km/hour$.