It is a riddle to make the equation 5+5+5=550 true by adding a single line. The solution is to change 5+5+5 = 550 to 545+5 = 550 or 5+545 = 550.
This is a very simple riddle because the plus sign must be transformed into a four for the solution to work. However, it can deceive many people because the equation initially appears to be incorrect.
A comparable conundrum could be 3+3+7=350. Fixing the equation by adding a straight line to one of the plus signs so that it becomes a four. These types of brain teasers are common in logic books and collections of mental exercises.
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