All solutions here are SUGGESTED. Mr. Teng will hold no liability for any errors. Comments are entirely personal opinions.
Depth drilled on the 10th day
Thus, it takes 40 days
I feel that (i) is fine, student just needs to read really carefully and beware if they are using the term or sum formulas.
For (ii), it is worth noting that some students still have no clue how to deal with inequalities when there is involved.