a) 18 * 3 5/6 = 18 * (3 + 5/6) = 18 * 3 + 18 * 5/6 = 54 + (18*5)/6 = 54 + (3*5) = 54 + 15 = 69
б) 6 3/16 * 4 = (6 + 3/16) * 4 = 6 * 4 + 3/16 * 4 = 24 + (3*4)/16 = 24 + 3/4 = 24 3/4
Ответ: a) 69; б) 24 3/4