Rate of medication elimination

[csands]

Homework Statement

If an antibiotic at 100mg eliminates itself from the body at a rate of 10% every 4 hours, after the first four hours the amount left in the body is 90%. Does the next 10% reduce by the remaining amount in the body (90 mg) or the original dose ingested (100 mg)?

Homework Equations

The Attempt at a Solution

 

[RTW69]

After the first 4 hours, 90mg of the antibiotic remain in the body. After the next 4 hours 81mg's of the antibiotic remains in the body etc.