You must add 45 mL of the 80 % alcohol to the 30 % alcohol to get a 35 % solution.
You can use a modified dilution formula to calculate the volume of 80 % alcohol
V1Ă—C1 + V2Ă—C2 = V3Ă—C3
Let the volume of 80 % mixture 1 = x mL. Then the volume of the final 35 % mixture 3 = (405 + x ) mL
(x mLĂ—80 % alc) + (405 mLĂ—30 % alc) = (405 + x)mL Ă— 35 % alc
80x + 12 150 = 14 175 + 35 x
45x = 2025
x = 2025/45 = 45