(;
GM[1]
SZ[19]
AW[dp][dd][qi]
AB[qd][od][pp]
TR[qi]
C[Problem 1. Black to play.  

How should Black attack the marked white stone ? ]
(;B[qk]
TE[2]
C[Correct Answer.  

The white stone is outnumbered three to one. In such cases, it is important for Black to press his advantage and immediately attack. In this position Black 1 is the correct direction; this move drives White toward Black's strong position at the top. ]
;W[qf];B[of]
C[Next, Black caps with 3 ... ]
;W[oi];B[ok];W[mi]
LB[kc:B][pq:A]
C[... the sequence to White 6 follows naturally. Black has build a moyo on the lower right in sente. He can next reinforce his position at the bottom with A or take up position at the top with B. <= ]
)
(;B[qg]
C[Failure.  

Black might make a checking extension from above with 1, but ... ]
;W[ql];B[qn]
C[... this stone is inefficient because it is too close to Black's strong enclosure above. In other words, Blacks stones are overconcentrated. <= ]
)
)