If 0.015 g of thyroid extract contains 25 mcg of thyroxine, how much thyroxine is contained in 15 mg of thyroid extract?

The answer is 25 mcg.

Let's first convert units.
1 g = 1000 mg
0.015 g = x
Make the proportion:
1 g : 1000 mg = 0.015 g : x
x = 0.015 g * 1000 mg : 1 g
x = 15 mg

Therefore, 0.015 g = 15 mg

Now make the proportion:
15 mg contains 25 mcg.
15 mg contains x.

15 mg : 25 mcg = 15 mg : x
x = 15 mg * 25 mcg : 15 mg
x = 25 mcg

Only one reactant yields 2 or more products. meaning you will only have 1 product and it will break down into 2 or more products. eg. h2o -> h2 + o2