(a) 1,3,4,6 = 6/(1-3/4)
(b) 1,5,5,5 = 5*(5-1/5)
(c) 1,6,6,8 = 6/(1-6/8)
(d) 1,8,12,12 = 12/(12/8-1)
(e) 2,2,11,11 = 11*(2+2/11)
(f) 2,2,13,13 = 13*(2-2/13)
(g) 2,3,5,12 = 12/(3-5/2)
(h) 2,4,10,10 = 10*(4/10 +2)
(i) 2,5,5,10 = 5*(5-2/10)
(j) 2,7,7,10 = 7*(10/7+2)
(k) 3,3,7,7 = 7*(3/7+3)
(l) 3,3,8,8 = 8/(3-8/3)
(m) 4,4,7,7 = 7*(4-4/7)
(n) 5,5,7,11 = 5*(7-11/5)
(o) 5,7,7,11 = 7*(5-11/7)