35 is 3/5 the way from 20 to 45
3/5 x 160 = 96 pints of 45% and
160-96 = 64 pints of 20% pure fruit juice needed to get 160 pints of 35%
Or the more traditional way to solve this is:
X+Y= 160
X = 160-Y
20X + 45Y = 160(35)
20(160-Y) + 45Y = 160(35)
20(160) - 20Y + 45Y = 160(35)
25Y = 15(160)
Y = 15(160)/25 = 3(160)/5 = 96 pints of 45%
X = 160-96 = 64 pints of 20%