A number is to be multiplied by the fraction 4/5. But Amit, by mistake multiplied it by 5/4 and obtained the number 81 more than the correct value. What was the original number?
200
240
180
120
Explanation:
Let the number be x.
According to the question,
=> x $\times$ 5/4 - x $\times$ 4/5 = 81
=> x $\times$ (25 - 16)/20 = 81.
=> x $\times$ 9/20 = 81.
=> x = 180.