120 -105 = 15nm in 3 minutes = 30nm in 6 minutes = 300nm in 60 minutes = 300kts.   On nav. computer: 60 (minutes) on inner scale next to 300 (kts) on outer scale; go to 105 (nm) on outer scale and read time to run next to it: 21 mins.  10:03 + 21 = 10:24.