Abag has a total of 120 notes in denominations of rs 2, rs. 5 and rs. 10. the total value of the notes in the bag is rs 760. if there were twice as many notes rs. 5 , the total value of the notes will be 960. find the number of rs 10 notes in the bag?

So there are three variables, rs2 label x, rs5 label y, rs10 label z
Lets write equeations using these variables. 
x+y+z=120  (A bag has a total of 120 notes)
2x+5y+10z=760  (total value)
2x+5*2y+10z=960  (twice as many rs5)

3 variables, 3 independent equations will give solution. There are a fair few ways to solve this.