If the circumference of a circle is decreased by 50% then the percentage of decrease in its area is:

As we know circumference of a circle will be directly proportional to its radius $$r_{1}$$ 
hence when it got half its radius will also get half
now area of a circle is directly proportional to square of its radius
hence when its radius gets half its area gets $$\frac{1}{4}$$th of its value
let us say area was A
after decrement it will be $$\frac{A}{4}$$
Decrement is of $$\frac{3A}{4}$$ which is 75% of its original value.