#### Answer

(a) There are 43 milligrams of phosphoric acid in a dosage of two teaspoons.
(b) There are 516 milligrams of phosphoric acid in the bottle.

#### Work Step by Step

(a) We can find the amount of phosphoric acid in a dosage of 2 teaspoons.
$21.5~mg/tsp \times 2~tsp = 43~mg$
There are 43 milligrams of phosphoric acid in a dosage of two teaspoons.
(b) $1~oz \approx 30~mL$
$1~tsp = 5~mL$
We can find the number of teaspoons of medicine in the bottle.
$4~oz = \frac{4~oz}{1}\times \frac{30~mL}{1~oz}\times \frac{1~tsp}{5~mL}$
$4~oz = 24~tsp$
We can find the amount of phosphoric acid in the bottle.
$21.5~mg/tsp \times 24~tsp = 516~mg$
There are 516 milligrams of phosphoric acid in the bottle.