Pandas mathematical operation, conditional on column value

Question

I need to make a mathematical operation which is conditional on the value in a second column. Here is the setup.

Given a simple dataframe (df):

Answer 1

where

I perform the math then mask it using pandas.Series.where by passing the boolean series df.col1.eq('B')

df.assign(math=df.col3.add(10).where(df.col1.eq('B')))

  col1  col2  col3  math
0    A     2     0   NaN
1    A     1     1   NaN
2    B     9     9  19.0
3  NaN     8     4   NaN
4    D     7     2   NaN
5    C     4     3   NaN