This is a percent of change problem.
Subtract the old value from the new value; then divide by the old value.
39.49 – 54.99 = -15.5.
Divide this by 54.99 to get -0.28.
The negative sign merely indicates that there was a decrease in price or value.
Present this in percent form.
You may disregard the sign since the question already implies that there was a decrease in price.