For 7/12 and 7/15 the LCD is 60.
4/12 + 11/15 = 1 1/15
165
11/12 + 1/3= 11/12 +4/12=15/12 your answer is 15/12 or 1 1/4
lowest common denominator of 4/5 and 7/15 would be 15 4/5=12/15
15
LCD(15, 11) = 165.
11 over 12 + 1 over 3= 11 over 12 + 4 over 12 = 15 over 12 or 1 and 3 over 12 or 1 and 1 over 4
180
120
LCD(3, 12, 15) = 60
9/11