i know to draw bar graph . iknow how to find scale of the bar graph but how to find the height of bar graph?

Consider the example given in lesson 5: Construction of Bar graphs in the study material.

In order to represent the given information with the help of a bar graph, the scale to represent money earned on the y-axis is taken as 1 unit length = Rs1500

Amount on money earned in January = Rs 9000

This is represent by a bar of length 6 units on the bar graph.  (Rs 9000 = 6 × Rs1500 = 6 units).

Similarly, the other amounts can be represented by bars of corresponding lengths.

If the amounts can not written as multiples of Rs 1500, then the length of the bars can be taken between the values taken on the y-axis. 

Amount on money earned in February = Rs 5000

This amount can not be written as a multiple of Rs 1500. But, Rs 4500 < Rs 5000 < Rs 6000, so the length of the bar representing  Rs 5000 can be taken somewhere between Rs 4500 and Rs 6000.