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Level 15 - NRIIGPTN
The answer to this puzzle is:
Words: TRINING, RIPING, TINING, TIRING, PINING, GRIPT, PIING, IRING, GIRT, GIRN, PRIG, TIPI, INTI, TRIG, GRIT, TING, GIN, NIT, NIP, GIT, PIN
Bonus words:
TRINING, RIPING, TINING, TIRING, PINING, GRIPT, PIING, IRING, GIRT, GIRN, PRIG, TIPI, INTI, TRIG, GRIT, TING, GIN, NIT, NIP, GIT, PIN