Ok smart math people, why does this work. u have 78 subtract your age add 40 and its year your born

He letter p on the number line below represents the number . (use the hyphen for negative numbers and write answer as a decimal, such as –7.2) number line from negative 7 to positive 7 in increments of 0.5 is shown. only the whole numbers are labeled. a point labeled p is placed at the eleventh tick mark to the left of 0.