(;AB[ab]AB[ba]AW[ac]AW[bc]AW[bb]AW[cb]AW[ca]AB[ad]AB[bd]AB[cd]AB[dc]AB[eb]AB[ea]AB[ed]TR[aa]LB[cf:12]LB[df:13]LB[ef:14]LB[ff:15]LB[cg:16]LB[dg:17]LB[eg:18]LB[fg:19]LB[gg:20]LB[bf:11]C[Japanese scoring, but suicide is allowed. Black wants to play at A19 as a ko threat. How much is this threat worth?FORCE]AP[goproblems]
(;B[bf]C[no])
(;B[cf]C[guess again])
(;B[df]C[no])
(;B[ef]C[nope])
(;B[ff]C[try again])
(;B[cg]C[no])
(;B[dg]C[Correct. If white responds, she has three black prisoners plus two points of territory = 5. If not, black has 10 territory, 5 prisoners, but has given three suicide stones to white = 12. 5 12=17RIGHT])
(;B[eg]C[noooo])
(;B[fg]C[nope])
(;B[gg]C[Close, but no cigar.]))