Question 547190
Let x be the number of litres of pure alcohol that is added. 

{{{x(1.00)+50(0.7)=(x+50)(0.8)}}}

(Note: each term consists of the number of litres times the percentage of alcohol)

{{{x+35=0.8x+40}}}
{{{0.2x=5}}}
{{{x=25}}}

Therefore, 25 litres of pure alcohol must be added.