Răspuns :
x=3a
15a<30⇒a<15⇒a∈{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}⇒x∈{0,3,6,9,12,15,18,21,24,27,30,33,36,39,42,45}
15a<30⇒a<15⇒a∈{0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15}⇒x∈{0,3,6,9,12,15,18,21,24,27,30,33,36,39,42,45}