P: Lee studies.
Q: Lee does well.
R: Logic is interesting.
S: Logic is easy.

3. Lee will do well only if logic is both easy and interesting. If she studies she will do well. Therefore, if logic is not easy she will not do well.

QRSAC, PQC |- SNQNC

1. QRSAC
2. PQC \show SNQNC
3. SN........hyp
4. RSANQNC...1 C subs
5. RNSNO.....3 O int
6. RSAN......5 AN subs
7. QN........6,4 C elim
8. SNQNC.....3-7 hyp elim

We need either a conditional or an indirect proof since there is no basic string (one without a binary signal) that will let us move forward for a direct derivation.