An office building housed some tenants and service workers.

We can let t = the number of tenants and s = the number of service workers. After the first action, in which 15 workers left, the number of tenants was unchanged. There were now (s - 15) service workers, and this was half the number of tenants. We can create the equation:

1/2t = (s - 15)

t = 2(s - 15)

After the second action, the number of service workers was still (s - 15), but 45 tenants had left, so we have (t - 45). The new relationship between the two groups was that there were now 5 times as many workers as tenants, so we have:

s - 15 = 5(t - 45)

Substituting 5(t - 45) for s - 15 in the first equation, we have:

t = 2(5(t - 45))

t = 10t - 450

450 = 9t

50 = t

Now, substituting 50 for t in the first equation, we have:

50 = 2(s - 15)

25 = s - 15

40 = s

Answer: C