we have the sequence
-20, -19, -18, ... 57, 58, 59, 60
so
a1=-20
a2=-19
a3=-18
a2-a1=-19-(-20)=1
a3-a2=-18-(-19)=1
the common difference is d=1
Remember the general formula
an=a1+d(n-1)
we have
an=60
a1=-20
d=1
n=?
substitute
60=-20+(1)(n-1)
60+20=n-1
80=n-1
n=81
therefore
the number of terms