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Two containers of different capacities were used to fill a vat with 140 liters of liquid. One container had a capacity of 1 liter and the other had a capacity of 4 liters. Given that the contents of 50 containers were used to fill the vat, how many times was each container used?
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The guiding system of equations is
x + y = 50, (1)
x + 4y = 140. (2)
Use substitution or elimination method to get the solution
x = 20, y = 30,
where x = how many times 1-liter container was used, and y = that of 4-liter container.