If she deposited $600 and had $621.60 after four years, then she earned: $621.60 - $600 = $21.60 in four years. It means in one year she earned $21.60 ÷ 4 = $5.40. Now we have to find out how many percent of $600 is $5.40:
$5.40 = x% from $600
5.4 = x/100 * 600
5.4 = 600x/100 / * 100 (both sides)
540 = 600x / ÷ 600 (both sides)
x = 0.9
So the interest rate was 0.9% annually.
Now we have to find out how much is 0.9% from $1100:
$1100 * 0.9% =
= 1100 * 9/1009 =
= 9900/1000 =
= 9.9
It means in one year she would earn $9.90. After 7 years she would earn $9.90 * 7 = $69.30, so she would have in her account:
$1100 + $69.30 = $1169.30
