The correct answer is (b) 30.
The mean of a set of numbers is the sum of the numbers divided by the number of numbers. In this case, we are given that $A = a – 5$, $B = b – 10$, $C = c – 25$, and $D = d – 40$. We are asked to find the mean of $A$, $B$, $C$, and $D$.
To do this, we first need to find the sum of $A$, $B$, $C$, and $D$. This is equal to $a – 5 + b – 10 + c – 25 + d – 40 = (a + b + c + d) – (5 + 10 + 25 + 40)$.
We are given that the mean of $a$, $b$, $c$, and $d$ is 50. This means that $(a + b + c + d) = 50 \times 4 = 200$.
Therefore, the sum of $A$, $B$, $C$, and $D$ is equal to $200 – (5 + 10 + 25 + 40) = 120$.
The mean of $A$, $B$, $C$, and $D$ is then equal to $120 / 4 = 30$.