One tap can fill a cistern in 3 hours and the waste pipe can empty the full cistern in 5 hours. In what time will the empty cistern be full, if the tap and the waste pipe are kept open together?

The value of b when

is:

1. 4

2. 36

3. 24

4. none of these

15 note books can be bought for ₹ 240. The number of note books that can be bought for ₹ 160 is:

1. 16

2. 10

3. 18

4. 15