Solution

 

SSTF idea (quick recap)

At every step, move the head to the closest unserved request (minimum absolute distance from current head).

Work it out (carefully, one hop at a time)

Current head = 58; pending = {45, 81, 185, 33, 175, 99, 150, 77}

1. Nearest to 58 → 45 (|58−45| = 13)
   Move: 58 → 45, distance = 13
   Pending now {81, 185, 33, 175, 99, 150, 77}

2. Nearest to 45 → 33 (|45−33| = 12)
   Move: 45 → 33, distance = 12
   Pending {81, 185, 175, 99, 150, 77}

3. Nearest to 33 → 77 (|33−77| = 44)
   Move: 33 → 77, distance = 44
   Pending {81, 185, 175, 99, 150}

4. Nearest to 77 → 81 (|77−81| = 4)
   Move: 77 → 81, distance = 4
   Pending {185, 175, 99, 150}

5. Nearest to 81 → 99 (|81−99| = 18)
   Move: 81 → 99, distance = 18
   Pending {185, 175, 150}

6. Nearest to 99 → 150 (|99−150| = 51)
   Move: 99 → 150, distance = 51
   Pending {185, 175}

7. Nearest to 150 → 175 (|150−175| = 25)
   Move: 150 → 175, distance = 25
   Pending {185}

8. Last → 185 (|175−185| = 10)
   Move: 175 → 185, distance = 10
   Pending {}

Sequence serviced

58 → 45 → 33 → 77 → 81 → 99 → 150 → 175 → 185

Total head movement

Add the distances: 13 + 12 + 44 + 4 + 18 + 51 + 25 + 10 = 177 cylinders

✅ Answer: 177