This is always true, and can be understood why through some algebra.
Let “x” represent your initial number.
STEP 1: Think of any number
x
STEP 2: Double it
2x
STEP 3: Add 10
2x + 10
STEP 4: Halve it
(2x + 10)/2
which equals
x + 5
STEP 5: Take away the number you originally though [sic] of in Step 1
x + 5 - x
which simplifies into
5
