ineed to get anaverage of 0.89 by adding any 4 numbers

Answer 1

(0.89 + 0.89 + 0.89 + 0.89)/4 = 0.89
or
{ (0.89 +a) + (0.89-a) + (0.89+2a) + 0.89 -2a) }/4 = 0.89
for any old a

so for example if a = 1
1.89 , -.11 , 2.89 , -1.11

Answer 2

which is better buy: $0.89 for 1 liter or $0.89 for 1 quart?