If all were general laborers, the payroll would be 93·34 = 3162 per day. The payroll is actually 3771 -3162 = 609 more than that.
Each equipment operator makes 29 more than a laborer, so there must be ...
... 609/29 = 21 heavy equipment operators
and 34 -21 = 13 general laborers
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If you write an equation for the number of heavy equipment operators (h), you get the same result in the same way. 34-h is the number of laborers
... 122h + 93(34 -h) = 3771 . . . equation for total daily payroll
... 29h + 3162 = 3771 . . . . . . simplify
... 29h = 609 . . . . . . . . . . . . . subtract 3162
... h = 609/29 = 21 . . . . . . . . divide by 29
