Answer
Oxygen: $n=10$
Lead:
first isotope $n=124$
second isotope $n=122$
Work Step by Step
According to the periodic table, the atomic mass of oxygen is 8.
From the question, oxygen isotope's mass number is 18.
The number of neutrons is given by:
$A=Z+n\rightarrow 18=8+n$
$n=10$
For lead (Pb), $Z=82$
in the first isotope $A=206\rightarrow n=124$
in the second isotope $A=204\rightarrow n=122$
