I will structure my answer in three parts:
1) Why I am answering this : I was myself looking to find the corresponding pixel of my cropped image.... and looking for the solution I landed on your question, only to realize that it is still unsolved! Hence I am sharing my solution after some brainstorming! Hope it helps !
2) Mistake in question : In the example you stated the chunk size = (10,10) according to this the coordinates will be 0 to 9 on both x and y.. per say x=10 or y=10 will not exist.
3) Solution : Let's say the pixel in main image is X=24,Y=27 i.e. (24,27) = A
Now by simple maths or else Arithmetic Progression we know that it will be part of sub image of size (10,10) starting from X=20, Y=20 i.e. (20,20)=S and ending at X=29, Y=29 i.e. (29,29)=E.
Now in this sub image, coordinates are :
[0, 0][0, 1]..........[0, 9]
[9, 0][9, 1]........[9, 9]
Now in this sub-image we want to find the original-image pixel for x=4, y=7 i.e. (4,7)=a
Thus to go from a(x)(or a(y)) to A(X) (or A(Y)), the relation is as follows:
A(X) = S(X) + a(x) =>> A(X) = 20 + 4 =24
A(Y) = S(Y) + a(y) =>> A(Y) = 20 + 7 =27
Thus , a ---> A is (4,7) ----> (24,27)
The inverse is also true!
All the best!!!