The Researcher will find 69 viruses, because 231 viruses are decayed in 7 hours, if a virus with a decay rate of 19% per hour and they start with 300 viruses.
Step-by-step explanation:
The given is,
                      A new virus with a decay rate of 19% per hour  
                     They start with 300 viruses
Step:1
                   Formula to calculate the viruses at decay rate,
                                                    ............................(1)
............................(1)
                  Where, y - No of viruses after given time
                              A - Viruses at initial stage
                               r - Decay rate 
                               t - No. of hours
                  From the given,
                               A = 300
                                r = 19% per hour
                                t = 7 hours
                 Equation (1) becomes,
                                             
                                                
                                                
                                                
                Viruses of 7 hours ≅ 69 viruses
                                                       ( or )
Step:1
         For 1st hour,
            = (No.of viruses in hour) - (No,of viruses in last hour × Decay rate )
               = (300)-(300 × 0.19)
               = 300 - 57
               = 243
        For 2nd hour,
               = (243)-(243 × 0.19)
               = 243 - 46.17
               = 196.83
       For 3rd hour,
               = (196.83)-(196.83 × 0.19)
               = 196.83 - 37.39
               = 159.4323
        For 4th hour,
               = (159.4323)-(159.4323 × 0.19)
               = 159.4323 - 30.2921
               = 129.1402
       For 5th hour,
               = (129.1402)-(129.1402 × 0.19)
               = 129.1402 - 24.5366
               = 104.6035
      For 6th hour,
               = ( 104.6035)-( 104.6035 × 0.19)
               =  104.6035 - 19.8747
               = 84.7288
      For 7th hour,
               = (84.7288)-(84.7288 × 0.19)
               =  84.7288 - 16.0984
               = 68.63 viruses
Result:
           The Researcher will find 69 viruses, because 231 viruses are decayed in 7 hours, if a virus with a decay rate of 19% per hour and they start with 300 viruses.