A glass is filled with water and the total weight is 220g. If the glass is halffilled with water, the total weight is 120g. What is the net weight of the glass?
There may be a logic way of doing it but the following is the longer "math" way.
G = weight of the glass
W = weight of the water.
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G + W = 220
G + 1/2(W) = 120
Two equations in two unknowns. Solve for G (and W if desired).
Post your work if you get stuck.