We have to use conservation of momentum here.
So m1v1=m2v2
where m1 and m2 are masses and v1 and v2 are velocitis
We can easily tranform our formula to get m2
\(m1*v1=m2*v2 \\ \frac{m1v1}{v2}=m2\)
Now we can substitute our data
m2=\(\frac{80*10}{8}=100\) [kg] - its our result