Answer
The life span shrinks by $16$ years
Work Step by Step
Sleep less than 6 - 8 hours:$\qquad-1$
Smoking cigarettes:$\qquad -15$
Adding these effects
$-1+(-15)$=$ \qquad$ use the rule for adding numbers of the same sign
$=-16$
The negative result means that the life span
shrinks by $16$ years
