I need a step by step solution for this. The book did not explain how to answer this question.

The following frequency distribution shows the monthly commissions earned by a sample of sales persons in the solar panel industry. What is the cumulative relative frequency, expressed as a percentage, for the "12,600 up to 14,400" class interval for the distribution? Monthly Commissions Class Frequencies $5400 up to $7200 7 7200 up to 9000 6 9000 up to 10800 12 10800 up to 12600 20 12600 up to 14400 40 14400 up to 16200 22 16200 up to 18000 31 18000 up to 19800 55