Trick:

If length is increased by 40% and breadth is decreased by 50% then % change would be 30%. Now again the process is repeated and now %  change would be 21%. Overall percentage change = 51%

Detailed solution:

Let the initial length and breadth be x and y respectively

Area = xy

Now length is increased by 40% and breadth is decreased by 50%

⇒ New length = x + (40% of x) = (140/100)x

⇒ New breadth = y – (50% of y) = (50/100)y

Now after repeating the same process

⇒ New length = (140/100)x + {40% of (140/100)x}

⇒ (140/100)x + (56/100)x = (196/100)x

⇒ New breadth = (50/100)y – {50% of (50/100)y}

⇒ (50/100)y – (25/100)y = (25/100)y

⇒ New Area = {(196/100)x} × {(25/100)y} = (49/100)xy