Below is a copy of a calendar for July 2015 .
Statement: The sum of any three dates in a row is always a multiple of 3.
(Do it algebraically and check if the given statement is true or false)
True
Let us consider any three dates in a row.
If we take the first number be x, then the second number must be 'x + 1' and the third one should be 'x + 2'.
Then the sum is
(x) + (x + 1) + (x + 2) = 3x + 3 = 3(x + 1)
We note that 3(x + 1) is a multiple of 3.
Hence the sum of any three dates in a row is always a multiple of 3.
Therefore the given statement is true.