The answer to this question would be: <span>85c
</span>Solubility is influenced by the temperature of the molecule. Higher temperature will result in a higher solubility. That is why brewing using hot water will be easier than using cold water. Higher temperature also means higher reaction rate.
Radioactive material obeys 1st order decay kinetics,
For 1st order reaction, we have
k =

where, k = rate constant of reaction
Given: Initial conc. 100, Final conc. = 6.25, t = 18.9 hours
∴ k =

= 0.1467 hours^(-1)
Now, for 1st order reactions: half life =

= 4.723 hours.
The answer for this is 26.6°c