Tips & Tricks GIVEN: Length of a rectangle is increased by 10% and its breadth is decreased by 20% .FORMULA USED: Area of rectangle = Length × Breadth CALCULATION: Suppose the length and breadth of the rectangle are ‘l’ and ‘b’ respectively. So, area of rectangle = lb Now, Increased length = 1.1l and decreased breadth = 0.8b ⇒ New area = 1.1l × 0.8b = 0.88lb Decrement in area = [lb – 0.88lb] = 0.12lb ∴ Percentage decrement in area = [0.12lb/lb] × 100 Smart trick Formula for successive percentage change of a% and b% = (a + b + ab/100) ∴ Percentage change in area = 10 – 20 – (200/100) = -12% (negative sign represents the decrement) Alternate Method We can solve this question by taking values 100 to ease our calculation.- Take length = breadth = 100 Hence, initial area = 10000 New length = 110 and new breadth = 80 New area = ∴ Required percentage