Solving $DEF+FEF=GHH$, $KLM+KLM=NKL$, $ABC+ABC+ABC=BBB$ She visits third class and is $8$ years old (you can imagine how ashamed I felt when I said so to her). I helped her with lots of maths stuff today already but this is very unknowable for me. Sorry it's in German but I have translated it :)

It's saying "Each letter represents a digit. Determine them". First question, what is "them"? The letters I guess?
How shall I determine them when they are unknown? Or is it simply $A=1, B=2, C=3, D=4, E=5, F=6, G=7, H=8, K=11, L=12, M=13, N=14$?
Alright...
With this we gave a) the first try:

It doesn't seem to make sense to set $A=1, B=2, ...$
Or we did something wrong.. Any ideas how this could be solved? :s

For the last equation, $3(100A+10B+C) = 111B$ and hence $100A+C = 27B$. Since $0 \le C \le 9$, you basically need a multiple of $27$ that is very close to a multiple of $100$. Only $27 \times 4$ works, so you know $A,B,C$.
For the second equation you just work from right to left, guessing $M$ and finding $L,K,N$ in order. For example if you guess that $M = 1$, you get that $L = 2$ and $K = 4$ and $N = 8$. But there is more than one solution, actually four in total!
For the first equation, nearly anything goes. We cannot impose that each letter stands for a unique digit, because there are too many letters.
