I'm confused on solving this question. The first step is to find the mean how ever I'm confused on when inserting numbers to the formula where are the numbers (15, 25,35,45,55) coming from ?

Mateo, Joaquin, and Carisma are a tree planting crew. Cumulatively, their tree planting is presented in the table below. Determine the standard deviation, assuming this is the population data. Class Interval Frequency 10 to under 20 3 20 to under 30 1O 30 to under 40 8 40 to under 50 2 50 to under 60 2 As the data is presented using a frequency distribution we will use the formula: _Zf(m-/1)2 'Zf So, we first need to determine the mean: Zn Yr _(3x1m+uox25p48x35p42x4spmzx53 3+10+8+2+2 _ 45+250+280+90+110 25 ll: _775 E