A number is first decreased by 15% and then increased by 20%. The number so obtained is 78 more than the original number. Calculation: Let the original number be x . After decreasing by 15%, the number becomes x−0.15x=0.85x After increasing by 20%, the number becomes 0.85x+0.20(0.85x)=0.85x+0.17x=1.02x According to the given condition: 1.02x=x+78 ⇒0.02x=78 ⇒x=78∕0.02 ⇒x=3900