Hello!
To start this answer off, and to show the way I was taught, I would like to made clear that Bryan's estimate is mathematically considered the "experimental value" and the actual cost is the "accepted value".
Considering these two factors, I can show the formula below.
Percent error = ([experimental value - accepted value] / accepted value) x 100
Now, I know that that seems a bit complex! But allow me to explain further.
We first will take the experimental value, also known as the estimate, and subtract that from the actual vaue, 850.
730 - 850 = -120
As you can see, the difference is a negative number. We are going to take this difference and use its absolute value. So, now instead of -120, we have 120.
Now we are going to divide the absolute value, 120, but the accepted value.
120 ÷ 850 = .14117647058
Finally, we will multiply the decimal value by 100.
Your final answer, as I believe, should be 14% if we are rounding to the nearest whole.