There are 75 students in a class, they all play at least one of the instruments namely violin, guitar, and harmonium. Further it is known that, 15 students play all the three instruments. 20 students play both guitar and violin, 30 students play both guitar and harmonium and 25 students play both violin and harmonium. A total of 50 students can play harmonium and a total of 45 students play violin. What is the number of students who can play only guitar?