Answer
a. after year 2016.
b. after year 2020.
c. after year 2020.
d. after year 2016.
Work Step by Step
a. Given $I=\frac{x}{4}+26$, let $I\gt33$; we have $\frac{x}{4}+26\gt33$, $\frac{x}{4}\gt7$ and $x\gt28$, which corresponds to year $1988+28=2016$; that is, after year 2016.
b. Given $N=\frac{x}{4}+6$, let $N\gt14$; we have $\frac{x}{4}+6\gt14$, $\frac{x}{4}\gt8$ and $x\gt32$, which corresponds to year $1988+32=2020$; that is, after year 2020.
c. Find the intersection of the results from parts (a) and (b); we have $x\gt32$, or after year 2020.
d. Find the union of the results from parts (a) and (b); we have $x\gt28$, or after year 2016.
