Five numbers, three darts each, doubles only to finish: ladder #215 asks for 12, 55, 61, 85 and 115.
12 is a straight double: D6. If the dart lands in the single by mistake you are left on 6, which still has a double finish with the darts you have left. 55 is a two-dart finish: 15 D20.
61 is a two-dart finish: T7 D20. 85 is a two-dart finish: T15 D20. 115 needs all three darts: T19 18 D20. If the first dart misses its treble, the board will show the new route straight away.
At best the five finishes need 10 darts out of 15. Your result counts the finishes you make first and the darts you used second. The middle finishes decide this ladder. Choose the route that leaves a double after a missed treble; the suggested routes do that wherever they can.