Answer: 20%
Step-by-step explanation: Notice that in this problem we're asked to find the interest rate not the interest so our answer will be a percent rate of interest.
Let's start this problem by writing the interest formula shown below.
Interest = principal · rate · time
Now we fill in the formula.
The interest earned is $300 so we substitute 300 into the formula.
The principal is the amount invested or $3,000.
We don't know the rate so we can use the variable r.
The time is 0.5 which is equivalent to 1/2
so we can substitute 1/2 in for t.
So we have the equation 300 = (3,000)(r)(1/2).
Simplifying on the right side of the equation, 3,000 · 1/2 is 1,500.
So we have 300 = 1,500r
Now to solve for r since r is being multiplied by 1,500, we need to divide by 1,500 on both side of the equation.
On the right side of the equation, the 1,500's cancel and we have r and on the left side of the equation we have 300 divided by 1,500 or 0.2.
This means that 0.2 = r.
Remember however that the interest rate is a percent so we need to change our decimal to a percent by moving the decimal point 2 places to the right to get 20%. So the interest rate is 20%.