The value of 150 kts didn't specify whether that was Indicated Air Speed (or 'Calibrated' Air Speed), True Air Speed or Groundspeed.
Assuming ISA, 150 KIAS is equivalent to 168 KTAS at 8000ft, but only 151 KTAS at 1000 ft. Using an average of 159 KTAS for the descent of 15.044 miles, that'll take you 5.68 minutes in which time you'll need to have descended 7000ft, making your descent rate target 1233 ft/min in still air.
Which is way over the top for what they're after. Use fractions and miles per minute, assuming that your speed remains a constant 150 kts groundpseed. 7000 ft in 15 miles at 2 1/2 miles per minute. What's 15 divided by 2 1/2? Exactly 6. So divide 7000 by 6 and you get 1167.
It's the sort of problem designed to test whether digi-yoof can reason, use fractions and work things out without the need for a calculator.
That said, West Coast's suggestion is an everyday pragmatic solution which is the sort of practical approach you'll learn later in life.