Answer
$2.4Kg$
Work Step by Step
We can find the mass of the system after collision as follows:
$\frac{2m_{\circ i}c^2}{\sqrt{1-v^2/c^2}}=m_{\circ f}c^2$
This can be rearranged as:
$m_{\circ f}=\frac{2m_{\circ}i}{\sqrt{1-v^2/c^2}}$
We plug in the known values to obtain:
$m_{\circ f}=\frac{2(0.240)}{\sqrt{1-(0.980)^2}}=2.4Kg$
