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$I = \$5810 - \$5000$ $I = \$810$ The simple interest earned is \boxed{\810}$.

ii) The annual interest rate paid by the credit union

$R = \frac{\$810}{\$5000 \times 3 \text{ years}}$ $R = \frac{\$810}{\$15000}$

The annual interest rate is $\boxed{5.4\%}$.

iii) The length of time it will take for Mr Adams' investment to be doubled, at the same rate of interest.

$\text{Doubled amount} = 2 \times P = 2 \times \$5000 = \$10000$

$I = \text{Doubled amount} - P = \$10000 - \$5000 = \$5000$

$T = \frac{\$5000}{\$5000 \times 0.054}$

The length of time it will take for the investment to be doubled is approximately $\boxed{18.52 \text{ years}}$ (rounded to two decimal places).

Question 5

i) If a washing machine was sold for $3960 during the promotion, what was the cost price?

Given:
Selling Price (SP) = $3960 Profit percentage = 12.5%

The selling price is the cost price plus 12.5% profit.

$\$3960 = X + (0.125 \times X)$ $\$3960 = X(1 + 0.125)$ $\$3960 = 1.125X$

$X = \frac{\$3960}{1.125}$ $X = \$3520$ The cost price was \boxed{\3520}$.

ii) During the Summer Sale, the washing machine's selling price was then reduced by 15% of the Easter sale price. What was the percentage profit or loss during the Summer Sale relative to the cost price?

Given:
Easter sale price (SP_Easter) = $3960 (from part i) Cost Price (CP) =$3520 (from part i) Reduction in Summer Sale = 15% of Easter sale price

$\text{SP\_Summer} = \$3960 - (0.15 \times \$3960)$ $\text{SP\_Summer} = \$3960 - \$594$ $\text{SP\_Summer} = \$3366$

$\text{Profit/Loss} = \$3366 - \$3520$ $\text{Profit/Loss} = -\$154$ Since the result is negative, it is a loss of $154.

$\text{Percentage Profit/Loss} = \left( \frac{-\$154}{\$3520} \right) \times 100\%$

There was a $\boxed{4.375\% \text{ loss}}$ during the Summer Sale relative to the cost price.