If B is the midpoint of line MN, then the length of MB is equal to the length of NB. Therefore, we can set up an equation:
MB = NB
We know that the length of MN is 15. We also know that B is the midpoint of MN, so:
MB + NB = 15
Substituting MB for NB, we get:
2MB = 15
Solving for MB, we get:
MB = 7.5
Therefore, the length of MB is 7.5.