Should be 20 Kg. The mass of 3 balls together is 15 kg implies that weight of one ball is 5 kg. Now dog is equal to 30 kg since there are 6 balls on balance. So deducting mass of two balls which equals (5+5=10 kg), it will be 20 kg.
Answer is D
Reason: Maya today is 18 years. So six years back she was 12 years old. At that time Namita was 48 (4 times Myaya's age), So Namita is 54 years ow (after 6 years). So Namita + Maya is 54 + 18 = 72