HKD1710 wrote:

Ron deposits $400 in savings account which gives compound interest semi-annually at the rate of 8% p.a. Approximately, how much interest does he receive in 2 years?

(A) $60

(B) $64

(C) $68

(D) $72

(E) $76

Ron's money earns 1.04 percent interest every six months over two years for a total of four payments. Total amount earned with compound interest** is given by

\(A = P(1 + \frac{r}{n})^{nt}\), thus

\(400(1.04)^4\)

The problem: the arithmetic and these answers. 1.04\(^{x}\) behaves much differently than .04\(^{x}\) does, so there are more numbers and more decimal places to multiply.

And these answer choices are wickedly close. I decided that a

yearly compounding rate of 1.08 * 1.08 was going to have to be close enough.

1.08 * 1.08 = 1.1664

1.1664 * $400 = $466.56

I had rounded down. The actual figure would be slightly more than that.

And I was annoyed enough by then that "pretty close" was good enough.*

Interest earned: $466.56 - 400 = $66.56 =

approximately $68

Answer C

I don't usually announce time taken. I think it matters here: 56 seconds. It's doable.

*After, I multiplied on a calculator to see the actual figures:

1.04\(^4\) = 1.16985856, * $400 = $467.943424

From above, by hand:

1.08 * 1.08 = 1.1664, * 400 = $466.56.

The first answer is exact and decisively points you to C.

The second isn't as decisive, but if you remember that you have rounded down using 1.08, Answer C is the right choice.

**

r is the decimal interest rate n is the number of periods

t is time in years

P is principal

Total amount

Divide .08 by 2 = .04.

Add .04 to 1 = 1.04 = rate of interest payment every 6 months. There are 2 pay periods, and two years = 4 payments.