In a mixture of concrete, there are four lb of cement mix for each pound of gravel. if the mixture contains a total of 115 lb of these two ingredients, how many pounds of gravel are there

For this case, the first thing we must do is define variables:
 x: pounds of gravel
 y: pounds of cement
 We now write the system of equations:
 x + y = 115
 y = 4x
 Solving the system of equations we have:
 x = 23
 y = 92
 
 there are 23 pounds of gravel

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