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?