According to question If a patient does not receive treatment for a certain injury within 2 hours after the injury.So after 2nd hour the patient's chance of survival starts decreasing by 15% for each hour.
By the end of the third hour, the patients survival rate is
$\frac{\left ( 100-15 \right )}{100}$=$.85$ i.e.$85$%
By the end of the four hour, the patients survival rate is
$.85\times (0.85) = 72.25$ i.e.$72.25$%
By the end of the fifth hour, the patients survival rate is
$.72\times (0.85) = 61.20$ i.e.$61$%
This process will continued like this .
So By the end of the four hour, the patients survival rate should be $72$.%.