I've never heard of a ? exit channel...
It does matter that the mass numbers do not match. In any nuclear decay or reaction, both the mass and the atomic numbers must add up properly on both sides of the equation. Also, it appears that you are incorrectly reading the reaction notation. What you have written is something commonly used to denote nuclear reactions where
21282Pb is the target
3115P is the projectile or entrance channel and ? is the exit channel. Another way to write is the first reaction you have is:
21282Pb+
3115P -> X + ?
Anyway, the reaction you have written does not tell you whether the exit channel is ?+ or ?-, so there are two possibilities. One is:
21282Pb(
3115P,
0-1?-)
24398Cf
or in more understandable notation:
21282Pb+
3115P ->
24398Cf +
0-1?-
If you really want to be correct, you can throw an antineutrino into the right side of that equation
Is the first reaction that you have written down correct though? Based on my understanding of nuclear reactions (So I may not be correct) I do not think that it is possible for a ? exit channel to exist and
21282Pb has a half-life of 10.64 hours, so there is no way that anyone will allow you to make a target out of that.