Question

A circular logo is enlarged to fit the lid of a jar. The new diameter is 50 per cent larger than the original. By what percentage has the area of the logo increased?

a. 50
b. 80
c. 100
d. 125
e. 250

Answer 1

It is easier to show th anser using fractions.

Do=original diameter
Ao=Original area
Df=Final diameter
Af=Final area

Df=(1+50%)Do=(1+1/2)Do=3/2*Do

Ao=pi/4*(Do)^2
Af=pi/4*(Df)^2=pi/4*(3/2*Do)^2=pi/4*9/...

Comparing the initial and final area:

Af/Ao=(pi/4*9/4*(Do)^2)/(pi/4*(Do)^2)

simplifying

Af/Ao=9/4=225%

Therefore the increment is 9/4-1=5/4=125%

Or equivalently: 225%-100%=125%

Therefore the answer is "D."

Note: there is calculate the increment in the difference not how many times the area changed.

Hope this helps ;)