Question

After deducting 10% from a certain sum, and then 20% from the remainder, there is Rs.3600 left. Find the original sum.

Answer 1

$let \: \: the \: original \: sum \: be \: \: x \\ \\ after \: deducting \: 10\% \: sum \: becomes \\ x -( x \times \frac{10}{100}) = x - 0.1x = 0.9x\\ \\ after \: deducting \: 20\% \: sum \: becomes \\ 0.9x -(0.9 x \times \frac{20}{100}) \\ =0.9 x - 0.18x = 0.72x \\ \\ given \: \: 0.72x = 3600 \\ x = \frac{3600}{0.72} = 5000$

Answer 2

Amount left = Rs 3600

Find the amount before deducting the 20% of the remainder:

100 - 20 = 80%

80% = 3600

1% = 45

100% = 45 x 100 = Rs 4500

Find the amount before deducting the 10%:

100 - 10 = 90%

90% = Rs 4500

1% = 4500 ÷ 90 = Rs 50

100% = 50 x 100 = Rs 5000

Answer: The original amount is Rs 5000