Question

The length of a rectangle is increased by 20 percent and breadth is increased by 30%.find the change in its area

Answer 1

from net effect formula
$x + y +xy \div 100$
let x = 20
y = 30
50 + 6 = 56 % change

method 2
length = 20% = 1/5 means length was 5 initially but after increasing 20% it became 6
breadth = 30% = 3/10 means breadth was 10 initially but after increasing 30% it became 13
so initial length = 5
initial breadth = 10
so initial area = 5×10= 50
new length = 6
new breadth =13
so new area =6×13=78
so % change in old area and new area
= [(78-50)/50] ×100
= (28/50) × 100
= 28×2 = 56%

Answer 2

Net Effect = 20+30+(20*30/100) %
                 = 50+6 %
                 = 56%