Answer:
Let x be the first number that the teacher wrote on the board and y be the second number. From the problem, we know that:
x + y = 58 (from Eric's calculation)
x - y = 28 (from Olivia's calculation)
To find the value of x, we can add the equations together:
x + y = 58
x - y = 28
2x = 86
x = 43
So the first number that the teacher wrote on the board is 43.