If you want to try to solve this problem in "words" rather than forming and solving equations then you could adopt this approach:-
After James had made 3 cakes there was [imath]\bf\small\frac{3}{4}[/imath] of the packet of flour left. This means that he must have used [imath]\bf\small\frac{1}{4}[/imath] of the packet to make those 3 cakes ([imath]\bf\small\frac{4}{4}[/imath] - [imath]\bf\small\frac{3}{4}[/imath] = [imath]\bf\small\frac{1}{4}[/imath]) and, therefore, he is using [imath]\bf\small\frac{1}{12}[/imath] of the packet for each cake ([imath]\bf\small\frac{1}{4}[/imath] divided by 3).
(NB: [imath]\bf\small\frac{4}{4}[/imath] and [imath]\bf\small\frac{12}{12}[/imath] are both ways to represent the whole packet flour; you do see that don't you?)
Therefore, after he has made another 5 cakes (3 + 5 = 8 cakes in total) he will have used [imath]\bf\small\frac{8}{12}[/imath] of the packet of flour (8 × [imath]\bf\small\frac{1}{12}[/imath] = [imath]\bf\small\frac{8}{12}[/imath]).
This means that he has [imath]\bf\small\frac{4}{12}[/imath] of the packet left ([imath]\bf\small\frac{12}{12}[/imath] - [imath]\bf\small\frac{8}{12}[/imath] = [imath]\bf\small\frac{4}{12}[/imath]).
So the 1,400 g of flour that remain after he has baked his 8 cakes is actually [imath]\bf\small\frac{4}{12}[/imath] (or [imath]\bf\small\frac{1}{3}[/imath]) of the whole packet.
Can you now work out how much flour James was using to make each cake?
(Hint: He now has enough left ([imath]\bf\small\frac{4}{12}[/imath]) to make another four cakes, doesn't he? 
)
Please come back and tell us your answer (and show us your working too, please).
Hope that helps. 
Yes, I can work out how much flour James was using to make each cake.
If 1,400 g of flour is 4/12 (or 1/3) of the whole packet, then the whole packet must have weighed 1,400 g x 3 = 4,200 g.
This means that James was using 4,200 g / 12 cakes = 350 g of flour per cake.
Therefore, the answer is 350 g.
Your explanation in words is clear and easy to understand. I especially like how you used the example of 4/4 and 12/12 to illustrate that they both represent the whole packet of flour. This helps to make the problem more concrete and easier to visualize.
