PROLOGUE: Principles of Problem Solving
1. Let r be the rate of the descent. We use the formula time ï„ distance
rate ; the ascent takes 1
15 h, the descent takes 1
r h, and the
total trip should take 2
30 ï„ 1
15
h. Thus we have 1
15 ïƒ
1
r ï„ 1
15 
1
r ï„ 0, which is impossible. So the car cannot go
fast enough to average 30 mi/h for the 2Âmile trip.
2. Let us start with a given price P. After a discount of 40%, the price decreases to 06P. After a discount of 20%, the price
decreases to 08P, and after another 20% discount, it becomes 08 08P ï„ 064P. Since 06P  064P, a 40% discount
is better.
3. We continue the pattern. Three parallel cuts produce 10 pieces. Thus, each new cut produces an additional 3 pieces. Since
the first cut produces 4 pieces, we get the formula f n ï„ 4 ïƒ 3 n ï‚¡ 1, n  1. Since f 142 ï„ 4 ïƒ 3 141 ï„ 427, we
see that 142 parallel cuts produce 427 pieces.
4. By placing two amoebas into the vessel, we skip the first simple division which took 3 minutes. Thus when we place two
amoebas into the vessel, it will take 60 ï‚¡ 3 ï„ 57 minutes for the vessel to be full of amoebas.
5. The statement is false. Here is one particular counterexample:
Player A Player B
First half 1 hit in 99 atÂbats: average ï„ 1
99 0 hit in 1 atÂbat: average ï„ 0
1
Second half 1 hit in 1 atÂbat: average ï„ 1
1 98 hits in 99 atÂbats: average ï„ 98
99
Entire season 2 hits in 100 atÂbats: average ï„ 2
100 99 hits in 100 atÂbats: average ï„ 99
100
6. Method 1: After the exchanges, the volume of liquid in the pitcher and in the cup is the same as it was to begin with. Thus,
any coffee in the pitcher of cream must be replacing an equal amount of cream that has ended up in the coffee cup.
Method 2: Alternatively, look at the drawing of the spoonful of coffee and cream
mixture being returned to the pitcher of cream. Suppose it is possible to separate
the cream and the coffee, as shown. Then you can see that the coffee going into the
cream occupies the same volume as the cream that was left in the coffee.
coffeeÂ