Simple Interest-10th grade-2

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Mr. Tan wants to invest in a bank. He wants to know what sum of money
will amount to $720 in 4 years at the rate at which a sum of money
doubles itself in 20 years.

Solution: From the second part, $100 amounts to $200 in 20 years.

So, interest on $100 for 20 years = $200 - $100 = $100

So, interest on $100 for 1 year = $(100/20) = $5

Now, from the first part,

Interest on $100 for 1 year = $5

So, interest on $100 for 4 years = $(5 * 4) = $20

So, when the amount is ($100 + $20) or $120, the principal is $100

So, when the amount is $1 the principal = $(100/120)

So, when the amount is $720 the principal = ${(100/120) * 720} = $600

The required sum = $600