# Area word problem

• Jan 18th 2010, 02:34 PM
lightstevoo
Area word problem
when mineral deposits formed a coating 1 mm thick on the inside of a pipe, the area through which fluid can flow was reduced by 20%. Find the original inside diameter.
area of circle = pi(r^2) diameter 2r

• Jan 18th 2010, 02:39 PM
Jhevon
Quote:

Originally Posted by lightstevoo
when mineral deposits formed a coating 1 mm thick on the inside of a pipe, the area through which fluid can flow was reduced by 20%. Find the original inside diameter.
area of circle = pi(r^2) diameter 2r

we just need to find $r$, then multiply it by 2 to find the diameter $D = 2r$.

The original area is $A_1 = \pi r^2$

When the radius is decreased by 1mm, the area is $A_2 = \pi (r - 1)^2$

But that is 20% of $A_1$. Thus we have

$A_2 = 0.2A_1$

$\Rightarrow \pi (r - 1)^2 = 0.2 \pi r^2$

Now solve for $r$ so you can answer the problem