A body having variable mass has initial mass of 5 kg. It is moving with a velocity of 5 m/s initially, after some time the velocity increases to 10 m/s. What will be the new mass if the body is moving in the absence of any external force ? 

2.5 kg

If no net external force acts on the system, the total momentum is conserved.

$m_1 v_1 = m_2 v_2$

$5 \text{ kg x } 5 \text{ m/s} = m \text{ x } 10 \text{ m/s}$

$\Rightarrow m = \frac{25}{10} \text{ kg }$

$\Rightarrow m = 2.5 \text{ kg}$