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It can be solved formally, but the answer is clear/self-evident from the description:
ANSWER. Martin has 550 mL initially; Oscar has 450 mL initially.
The formal solution is below.
From the condition, you have this system of equations
M + O = 1000 mL (1)
M - 50 = O + 50 (2)
Add the equations. You will get
2M + O - 50 = 1000 + O + 50.
Simplify
2M = 1000 + 50 + 50
2M = 1100
M = 1100/5 = 550.
Then from equation (1)
O = 1000 - M = 1000 - 550 = 450.
as it was predicted at the very beginning.
Solved, answered, explained and completed.