I think this your question is not very complete and therefore the right answer can not be obtained.
But I would rather give you some highlights on this area of study;
In radioactivity we have emissions such as alpha, beta and the gamma rays.
These radiations have certain amounts of kinetic energies that they are being accompanied with during their emission.
your answer is incomplete but I understood your question.
The KE of alpha particle is given as 48Mev and Q value is given as 50Mev.
you have to find the mass number of the nuclei
KE of alpha particle = [A-4/A] Q where A is the mass number.
hence, 48=[A-4/A] 50
48A = 50A -50* 4
48A = 50A -200
2A = 200
A=100 is the answer
i think this may be helpful for you.
se iokkk @gmail.com