A methematician just would add the numbers and give the result in an ordinary way: 15.4 + 28. = 42.4; while the scientist would say 42
Justification:
A scientist would take into account the number of significant figures.
15.4 has three significant figures while 28. has only two significant figures.
Then it does not make sense to report the result with three signficiant figures and the scientist wiould round 42.4 to 42.
