The frequency distribution of daily working expenditure of families in a locality is as follows
 

Expenditure in Rs (x)	0-50	50-100	100-150	150-200	200 - 250
No. of families (f ):	24	33	37	b	25

If the mode of the distribution is Rs 140, then the value of b is

Frequency distribution is given as
 

Clearly, modal class is 100-150, as the maximum frequency occurs this class

Given, mode=140

We have Mode= $\ell +\frac{f_0-f_1}{2f_0-f_1\mathit{-}{f}_{\mathit{2}}}\times i$ Where
$\ell =100,f_0=37,f_{-1}=33,f_{1=b}$
$i=50\text{ Thus, we get }140=100+\left[\frac{37-33}{2\left(37\right)-33-b}\right]\times 50$
$=100+\left[\frac{4}{74-33-b}\right]\times 50=100+\frac{200}{41-b}\Rightarrow 5740=4300+40b\Rightarrow b=36$