If the mass of a body is doubled and its velocity becomes half, then the linear momentum of the body:

Let the mass of the body be $\mathrm{m} \mathrm{kg}$ and the velocity of the body be $\mathrm{v} \mathrm{m}/\mathrm{s}$.

$\therefore$ Momentum $\left(p\right)=m\times v$ 

Now, we will double the mass of the body, and the velocity of the body is reduced to half. 

So, the new mass of the body $\left(m_1\right)=2m$

And, the new velocity of the body $\left(v_1\right)=\frac{v}{2}$

Now, new momentum,

$\Rightarrow p_1=m_1\times v_1$

$\Rightarrow p_1=2m\times \frac{v}{2}$

$\Rightarrow p_1=mv$

Hence, p₁ = p (it remains the same). 

Therefore, the momentum will remain the same.