The LCM of 15 and 18 is 90
90 = 15 x 6 = 18 x 5
18
27
3
any multiple of 90
90
15-18 year olds go to the Lycée (masc.) which is the equivalent of senior high schools.
18
If that's 18 and 115 then the greatest common factor is 1
15 with remainder 5.
18 R 15