The outer circumference of a circular race-track is 528 m. The track is everywhere 14 m wide. Calculate the cost of levelling the track at the rate of 50 paise per square metre (Use π = 22/7)
It is given that the outer circumference C of circular track is 528 m.
We know that the circumference of circle of radius r is
$C=2 \pi r$
Substituting the value of C,
$528=2 \times \frac{22}{7} \times r$
$528 \times 7=44 r$
$r=\frac{528 \times 7}{44}$
$r=84 \mathrm{~m}$
Thus, the radius of outer circle is.
Since circular race track is 14 m wide everywhere. Then
radius of inner circle $=$ radius of outer circle $-14$
$=84-14$
$=70 \mathrm{~m}$
We know that the area of circle of radius $\mathrm{r}$ is $A=\pi r^{2}$
So, Area of outer circle $=\frac{22}{7} \times 84 \times 84$
Area of inner circle $=\frac{22}{7} \times 70 \times 70$
Now,
Area of circular track = Area of outer circle-Area of inner circle
$=\frac{22}{7} \times 84 \times 84-\frac{22}{7} \times 70 \times 70$
$=\frac{22}{7}(84 \times 84-70 \times 70)$
$=\frac{22}{7} \times 2156$
Area of circular track $=6776 \mathrm{~m}^{2}$
It is given that,
The cost of levelling the track per square meter $=$ Rs $0.50$
So, The cost of levelling the track 6776 square meter $=$ Rs $0.50 \times 6776$
$=\operatorname{Rs} 3388 /-$