Answer: 4 ways
To answer this question, you need to make a list based on how much "10 group" of rock can be made.
The possible way to group it should be:
0 pcs of 10 group + 38 pcs of 1 group
1 pcs of 10 group + 28 pcs of 1 group
2 pcs of 10 group + 18 pcs of 1 group
3 pcs of 10 group + 8 pcs of 1 group