How to show quantisation error for frequency coefficient

Question

I have the following question from an exam which reads:

For the second question my answer is:

24 5 2 3
 9 2 1 3
 2 3 2 2
 1 1 1 0

I am unsure of what formula to apply to each value to get the quantisation error. Can anybody tell me??

Answer 1

When the data is reconstructed from the quantized coefficients, the resulting coefficients will be

240 100  60 120
180  60  40 210
 60 120 140 200
 40  70 100   0

Thus the (signed) quantization errors are

 -3   2  -3  -3
 -7  -5   8  32
 -8  -1   9   2
-14 -23  38 -81