A radioactive sample decays to half in 1 day. What percent of initial sample decays in 3 days?

Half life of sample is $=$ 1 day, so in 3 days, it goes through 3 half - lives.

So, it remains, ${\left(\frac{1}{2}\right)}^3=\frac{1}{8}th$ of initial sample after 3 days.

decayed amount $=(1-\frac{1}{8})\times 100\%=87.5\%$