Answer is 2. Why? 公因數, or greatest common factor, of LAB and 72 is 12. 72: 1*72, 2*36, 3*24, 4*18, 6*12, 8*9 1,2,3,4,6,8,9,12,18,24,36,72 12*9=108 is the lowest number for L=1 (A=0 and B=8). 108: 1*108, 2*54, 3*36, 4*27, 6*18, 9*12 1,2,3,4,6,9,12,18,27,36,54,108 (公因數 with 72 = 18) 120: 1*120, 2*60, 3*40, 4*30, 5*24, 6*20, 8*15, 10*12 1,2,3,4,5,6,8,10,12,15,20,24,30,40,60,120 (公因數 with 72 = 12) Therefore, LAB must be 120 since 12 is the greatest common factor between 120 and 72. A=2 and B=0 and 2+0=2 :-)