Here is the question:

An unknown radioactive element decays into non-radioactive substances. In

days the radioactivity of a sample decreases by

percent.

(a) What is the half-life of the element?

(b)How long will it take for a sample of

mg to decay to

mg?

I've tried it several times and I keep getting the same answers which are wrong

I used

I integrate this and got

.

I used

which gave me the equation

.

I then plugged in

for y and 400 for t from the question. this gave me k= 0.0025. giving me the equation

.

I then set

and solved for t to get the half life. Which gave me the answer 277 days.

What am I doing wrong?