Done! Suppose the invoice is for P pounds and p pence.
This is (100P + p) in pence.
Reversed we have (100p + P) pence.
We are told (100P + p) / 4 = (100p + P)
That is 100P + p = 400p + 4P
Simpifying we get 96P = 399p
Cancelling by 3 we have 32P = 133p
This can only hold true (within the constraints) if we have
P = 133 and p = 32
So the invoice is for �133.32 as stated!