Answer:
90/900 × 100,000.
Step-by-step explanation:
Mortality ratio may be defined as the ratio used to define the number of deaths in the cohort or group.
The mortality ratio can be calculated by the following formula:
Mortality ratio = Number of death from specific reason/ total number of death × total number of individual.
The mortality ratio due to roller skating =

Thus, the answer is 90/900 × 100,000.