If the first line in the Lyman series has wavelength $\lambda$, then the first line in Balmer series has the wavelength

The half-life of a radiocative isotope is 30 h . How long will it take to get reduced to $12.5 \%$ of its initial amount?

The shortest wavelength in Lyman series is $912 \mathop {\rm{A}}\limits^{\rm{o}}$. Then, the longest wavelength in the series must be

Half-life of radioactive sample is 24 h . If a newly prepared radioactive sample shows 4 times the allowed and safe value of radio activity, the minimum time after which one can work safely with the source is