Question

3 kg onions by using 5 kg and 2 kg Weights

Answer 1

Take the beam balance.
On one side place the 5kg weight, on the other side place the 2kg weight.
Now keep placing onions on the side with the 2kg weight until both sides are balanced.
This means, 5kg= 2kg + mass of onions
This implies the mass of onions would be equal to 3kg.