When deformation is maximum, both the particles are moving with same velocity. So, applying momentum conservation,
m1v1+m2v2=m1v′1+m2v1′
v′1=m1v1+m2v2m1+m2
Applying energy conservation,
12m1v21+12m2v22
=12(m1+m2)(v1′)2+ΔU deformation
⇒ΔU deformation=12m1m2(m1+m2)×(v1−v2)2=1003
⇒v1−v2=10 m/s
Final answer: 10