Let age of ani =a, biju=b, cathy =c, dharam =d.
there are 2 cases, lets say Ani is elder than Biju , case 2 would be Biju is elder than Ani,
Try that too, but lets take in case 1.
a-b = 3
4a - b = 60
- a - b = 3
3a = 57
a= 19, as biju is smaller than ani, b=16, d=2a, so d=38, c=b/2 so c=8years.
You could take ani as smaller than biju, but u would get it wrong and dharam is father of ani , so will be bigger than biju and cathy.
now d-c = 30, as 38-8= 30
so you may get it wrong on first attempt , but see to it that u try both cases.