Err, the simple way is surely to work out the still air times then change them by whatever percentage the head/tail wind is of your TAS.
TAS 90kts = 2/3nm/min so 15 miles = 10 mins
HWC = 12 kts so about 13% increase on still air time, = 11.3 mins
Or if you prefer, just use 10% of g/s in 6 mins. GS=75kts gives 7.5nm in 6 minutes. 21nm is just less than 3 lots of 6 minutes.
Personally I like using the DI to solve all my drift and H/TWC sums "on the fly" - just look at it, find the wind and use ratios to do the MDR assessments - trivially simple, totally accurate, no gadgets needed and no maths.
HFD