Here let take a=10 , c= 2 (for easy calculation we want to keep the value of b as integer and not fraction)

so b =\(\frac{(4 * 10)}{(2)}\)

or b= 20

Now a is halved = \(\frac{10}{2}\) = 5

and c is doubled = 2+2 = 4

Now b= \(\frac{(4 * 5)}{(4)}\) = 5

Therefore percentage decrease = \(\frac{(5-20)}{20}* 100\) = -75% i.e decrease
_________________

If you found this post useful, please let me know by pressing the Kudos Button

Explanation

Try plugging in values for a and c. Let a = 6 and c = 2.

Then b= =3. If a is halved, equal it will 3. If c is doubled, it will equal 4. So now, b= =3.

Because the question is looking for a percentage decrease, apply the percentage change formula:

\(\frac{12-3}{12}\times 100=\frac{9}{12}\times 100=75%\).
_________________

Sandy
If you found this post useful, please let me know by pressing the Kudos Button

Try our free Online GRE Test