If we consider four numbers, the average of first three is 18 and that of last three is 19. If the last number is 16, find the first number?

Let numbers be a, b, c, d (with d = 16). Average of first three = (a+b+c)/3 = 18, so a+b+c = 54. Average of last three = (b+c+d)/3 = 19, so b+c+d = 57. Then (b+c+16) = 57, giving b+c = 41. Substituting: a + 41 = 54, so a = 13. Option B would give average of first three = 19. Option C would give average of first three = 17. Option D would give average of first three = 17.67.