Answer
The three pages are numbered $19,20,$ and $21.$
Work Step by Step
$\bf{\text{l. Familiarize yourself with the problem.}}$
If the first page number is $x$,
then the next two are $x+1$ and $x+2$.
We need the sum of $x,\ x+1$, and $x+2$ to be $60$.
$\bf{\text{2. Translate to mathematical language. (This often means writing an equation.)}}$
$x+x+1+x+2=60$
$\bf{\text{3. Carry out some mathematical manipulation. (This often means solving an equation.)}}$
$3x+3=60$
$3x=57$
$x=19$,
The next two are 20 and 21.
The three pages are numbered $19, 20,$ and $21.$
$\bf{\text{4. Check your possible answer in the original problem.}}$
Is $19+20+21=60$?
Yes.
$\bf{\text{5. State the answer clearly, using a complete sentence.}}$
The three pages are numbered $19, 20,$ and $21.$