115 students could sign up to hear three different speakers for career day. 70 students heard the nurse speak, 52 heard the firefighter, and 78 heard the webmaster. Some students heard more than one speaker. The results are shown in the table below.
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Number of students | Speaker
15 | All three
20 | Nurse and firefighter
30 | Webmaster and nurse
12 | Firefighter only
How many students signed up for Webmaster only?