Standardised Mortality Ratio

 The standardized mortality rate (SMR) is the ratio of the number of deaths observed in a population over a given period to the number that would be expected over the same period if the study population had the same age-specific rates as the standard population

SMR = (Observed death/Expected death)*100

In this question

Total observed death = 8+9+13 = 30

Total expected death = (2/1000)*3000 + (3.5/1000)*2000 + (6/1000)*2000 = 25

Therefore standardised mortality ratio = (30/25)*100 = 120