In the Book Of Why by Judea Pearl there is the mention of the dependence of weights of guinea pig pups on gestation period, as explored by Sewall Wright. The following is the causal diagram provided -
In the above diagram,
X = weight of pup in grams
P = Gestation period in days
Q = Utero growth rate
L = litter size
A and C = exogenous factors
We are interested in finding the impact of changing P on X in terms of grams/day, depicting the additional weight gain for each extra day of gestation.
The following are the path coefficients for each causal relation -
$L{\rightarrow}Q=l$
$L{\rightarrow}P=l'$
$Q{\rightarrow}X=q$
$P{\rightarrow}X=p$
Then the book says that, the influence of $P$ on $X$ can be found by $p+(l{\times}l'{\times}q)={\,}\text{rate of change of }X\text{ per day in grams/day}$.............$(i)$
My query is, what do the path coefficients denote exactly, in terms of their mathematical formulation? How are changes $\Delta P$ and $\Delta L$ related in terms of $l'$? Based on the definition of the coefficients, how is equation $(i)$ derived?