Answer: 30 minutes
Work:
If it takes her 12 minutes to knit 10 rows (total) how long will it take to knit 35 rows (total).
=

420 = 10x
42 = x
X represents how long it takes her to knit 35 rows, but the question is asking for the next 25 so simply just subtract 12 min from 42.
42 - 12 = 30 minutes