How to calculate with respect to money?? if A and B undertake to do a piece of work for Rs.108 . A can do it in 5 days while B can do in 7 days . if both of them finished the work together , how much shell B get s? I am confusing with money here !! How to calculate with respect to money??

We should think of terms of rates here, not times. And we could base the payment on the rates.

$A$ can do $1$ job in $5$ days, so $A$'s rate of work is $1/5$.

$B$ can do $1$ job in $7$ days, so $B$'s rate of work is $1/7$.

Their combined rate is $\frac15+\frac17=\frac{12}{35}$.

$B$'s fraction of that work is $(\frac17)/(\frac{12}{35})=\frac5{12}$.

Therefore, $B$ should get $\frac5{12} \cdot 108=45$ Rs.

This is the same answer as Jasper's but with more detail (and more confidence, I hope).