Concept: The question tests on application of Mixtures
Solution:
4 gallons of 90% glycerine contains 0.9 * 4 = 3.6 gallons of glycerine and 0.4 gallons of water.
Let us add x gallons of water to make it a 75% glycerine (25% water solution)
Therefore 3.6/0.4+x=75/25
3.6 = 3(0.4 + x)
3x = 2.4
x = 0.8
Option E