Answer
$2\;mL$.
Work Step by Step
Let the required quantity of the solution is $=x$.
Quantity of the solution is proportional to the quantity of streptomycin sulfate.
We can write.
$\Rightarrow \frac{2.5\;mL}{1000\;mg}=\frac{x}{800\;mg}$
Cross multiply.
$\Rightarrow (2.5\;mL)(800\;mg)=x(1000\;mg)$
Divide both sides by $1000\;mg$.
$\Rightarrow \frac{(2.5\;mL)(800\;mg)}{(1000\;mg)}=x$
Simplify.
$\Rightarrow 2\;mL=x$.
