Well, a right angle is 90 degrees
Half of 90 is 45
Three 15 degree angles make 45 degrees
another one makes 60
15+15+15+15=45
another 15 equals 60
15+15+15+15+15=60
another equals 75
15+15+15+15+15+15=75
90-75=15
so a 75 degree angle is one 15 degree angle short of a right angle