Cognition: Mental Representations, Problem Solving, and Decision Making - Foundations for Designing User-Centered Systems - page 174

Table 6.2 Puzzles that require backtracking to solve
Goat, cabbage, and wolf
You have a goat, a cabbage, and a wolf that you have to carry across a river. Your boat can only
hold you and one other object. If you leave the goat with cabbage, it will eat the cabbage. If
you leave the wolf with the goat, it will eat the goat. How do you ferry them across the river
without anything getting eaten?
Missionaries and cannibals
There are three missionaries and three cannibals that have to cross a river. Their boat can only
hold two people. If there are more cannibals on a side of a river than missionaries, the
cannibals will eat the missionaries. How do they ferry themselves across the river without
anyone getting eaten?
Fig. 6.8 You are in a room and have two strings to tie together (imagine you are MacGyver, a
US TV personality who has to solve odd problems using odd things). You have a hammer, some
tacks, a candle, and a box of matches. The two strings are too far apart for you to reach both of
them when they are hanging down, but close enough to tie together if you have each in a hand.
How do you tie them together?
series of problems using water jugs. In these problems, there were three jugs and a
target amount of water to get. For example, one problem was to get 3 gallons using
1-, 4-, and 5-gallon buckets (fill the 4-gallon, empty into the 1-gallon, and be left
with 3 gallons in the 4-gallon bucket). Over several trials they learned a strategy
that worked (such as fill the first bucket, empty into the second bucket, fill the first
bucket, empty into the third bucket, pour the second and third buckets together).
When given a new problem that was solvable with a more efficient strategy (such
as using 1-, 5-, and 7-gallon buckets, get 2 gallons by filling the 7-gallon, and
emptying into the 5-gallon to get 2 gallons), they would continue to use the less
efficient strategy (more pours), or if given a problem that required a different
strategy, they would first try the old, unworkable strategy.
Functional fixedness and Einstellung both arise when people do not make full
use of their knowledge, but are influenced too much by previous successes. As you
design systems you should be mindful of what previous knowledge and mental