Question 23368
Let's follow the antiseptic in start, add, and finish.
Let amount of 60% anti. be "x"
Then amount of 90% anti. is "50-x"
Amount of anti in 60% is 0.60x
Amount of anti in 90% is 0.90(50-x)= 45-0.9x
EQUATION:
Amt. in 60% + Amt. in 90% = Amt in 78%
 Amt. of anti in 60% + Amt. in 90% = Amt. in 78%
      0.60x + 45-0.90x = 0.78(50)ml
        60x + 4500-90x = 78(50)
                -30x = -600
                   x = 20 ml
              50-x = 30 ml

Cheers,
Stan H.