There are 28 street lamps in this street. Every two street lamps are equipped with a light bulb. We can calculate mathematically how many light bulbs there are in this street.

First of all, we can divide these 28 street lamps into 14 groups, with two street lamps in each group. Each group of two street lamps is equipped with a light bulb, so there are 14 light bulbs on this street.

However, there is a hidden condition of this problem, that is, after installing a light bulb for every two street lamps, an extra light bulb needs to be added to replace the broken light bulb. So in fact, the number of light bulbs needed in this street should be 14 light bulbs plus one extra light bulb, which is 15 light bulbs.

Therefore, there are 15 light bulbs in this street.