(1/2+1/5):2-3/10 = (5/10 + 2/10) : 2 - 3/10 = 7/10 : 2 - 3/10 = 7/20 - 3/10 = 7/20 - 6/20 = 1/20 = 0.05
(1/2+1/5):2-3/10 = (5/10 + 2/10) : 2 - 3/10 = 7/10 : 2 - 3/10 = 7/20 - 3/10 = 7/20 - 6/20 = 1/20 = 0.05