Explanation:

As per the information provided in the question:
Z = X + 8 ….(i)
P + Q = 21 ….(ii)
Q = P – 5 ….(iii)
X + Y = 28 ….(iv)
P = X + 5 ….(v)
We need to find the value of P + Q + X + Y + Z
Using ii and iii, we get:
P + Q = 21
Or P + (P - 5) = 21
Or P = 13
So, Q = P – 5 = 13 – 5 = 8
Using v, we get:
P = X + 5
Or X = P – 5 = 13 – 5 = 8
Using i, we get:
Z = X + 8
Or Z = 8 + 8 = 16
Using iv, we get:
X + Y = 28
Or Y = 28 – 8 = 20
So, P + Q + X + Y + Z = 13 + 8 + 8 + 20 + 16 = 65
So, total cost of all the notebooks = 65 × 40 = Rs. 2600