Question 26087
#1. Follow the acid.
Start with 80%(5)
Add 2%(x); x because the amount of fluid added is unknown
End up with: 10%(5+x)
EQUATION:
80%(5)+2%(x)=10%(5+x)
Multiply through by 100 to get:
80(5)+2x=10(5+x)
400+2x=50+10x
8x=350
x= 43.75ml (amount of 2% fluid you need to add) 
Cheers,
Stan H.