A = fruit juice 40%
B= 90% juice
then
30 gallons + A = 55• C/100
30 gallons + N • (40/100) = (30 + N)• (55/100)
Now solve this equation, for N
3000 + 40N = (30+N) •55
then
3000 - 1650 = 55N - 40 N = 15N
1350 = 15 N
then
N= 1350/15 = 90
Then answer is
Should be mixed 90 gallons of 40% fruit punch