The average weight of 10 persons increases by 3 kg when a new person comes in place of one of them weighing 55 kg. The weight of the new person is:

85

Let the average of 10 people be x kg

And the weight of the new person be y kg

Sum of 10 people weight is

 10× x = 10x kg

When the person is replaced.

 10x + y - 55 = 10 × (x + 3)

 10x + y - 55 = 10x + 30

 y = 55 + 30 = 85 kg

∴ The weight of the new person is 85 kg.