If there are 15 people each has to pay Rs 20 , this means 
total expenditure is Rs 300.
For 20 people total expenditure is Rs 360.
With increase in 5 person increase in expenditure is Rs 60 .
Therefore with increase of 20 person total expenditure 
increase is Rs 240.
Hence total expenditure is Rs 400.
Total no of person is 40.
Amount to be paid by each person is Rs 12.50.

for 15 people = Rs.300
  i.e 15*20 = 300
  for 20 people = Rs.360
  i.e 20*18 =360
  increase in 5 people => Rs 60
  for 1 = 60/5 =12
  for 20 = 12* 20 = 240
  so for 40 people = 360 + 240
                   = 600
  one person has to pay =Rs. 15
  i.e 600/40 = 15

for 15 = 300
for 1 = 20
for 20 = 360
for 1 = 18
for 20 * 20 = 400 actual
   but  = 360
dedution = 40
for 20 = 40
for 1 = 2
for 40 = 40 *2 = 80
total = 40* 20 =800 - 80
               =720
for 1 = 720/40 = 18

FOR 15 = 300
FOR 20 = 360
FOR 5 INCREASE = 40 DISCOUNT
FOR 1 = 8
FOR 20 = 20*8 = 160
TOTAL =40*20 =800 - 160 = 640
FOR 40 = 640
FOR 1 = 640 / 40
      = 16

FOR 15 = RS.20
FOR 20 = RS.18
FOR 25 = RS.16
FOR 30 = RS.14
FOR 35 = RS.12
FOR 40 = RS.10