Question

A 4 kg block ‘A’ and a 3 kg block ‘B’are pushed together against a spring, and are tied with a cord. When cord breaks, the 3 kg block moves to the right at 18km/s. What is the velocity of the 4 kg block? 

Answer 1

$$m_1v_1=m_2v_2\\3(18)=4v_2\\v_2=13.5\frac{km}{s}$$