Question

11/2 cup of black beans provides 15% of potassium you need daily. You must get the remaining 2890 mg from other sources. How many milligrams of potassium should consume daily?

Answer 1

The 11/2 cup of black beans is extraneous info, you just need to know that it provides 15% of the potassium you need daily. With that info, you know that you need 85% more to fulfill your daily value. If we say x is the total amount of potassium needed, then we can set up the equation:
x * 0.85 = 2890
Then we can solve for x by dividing both sides by 0.85. The answer, then, is 2890/0.85 = 3400 mg.

Answer 2

0.85p=2890

Where 0.85 is the percentage of the remaining amount of potassium needed (100%-15%=85%=0.85)
p is the total amount of potassium needed per day
2890 is the remaining amount of potassium needed

0.85p=2890
------- -------
0.85    0.85
p=3400

You should consume 3400 mg of potassium per day.