With each pouring, 2% of the mixture is displaced.

Hence, 98% of the blood remains.

Afternpourings, there are: B = 50(0.98)^n gallons of blood in the tank.

We can see that B will never equal 0 (in a finite number of pourings).

If B<0.01%, we have: 50(0.98)^n<0.0001 . . .at most0.01%

Then: n = ln(0.000002) ÷ ln(0.98) = 649.5348951

Therefore, it will take 650 pourings.