1. Bill decided to buy six books on history instead of four books on politics. It follows, then, that

the opportunity cost of each history book was

books on politics.

2/3 books on politics

Step-by-step explanation:

The opportunity cost of something is the next best alternative that you would have taken if you did not instead go for the option that you took.

Opportunity cost here is the number of books on politics that Bill had to give up to get a single book on history.

= Number of politics books / Number of history  books

= 4 / 6

= 2/3 books on politics

Bill gave up 2/3 books on politics to get a single history book.

The answer is in the image 

The solution is in the following image

Here,

tip=18%of $32

tip=(18/100)*32

=0.18*32

=$5.76

Total payment=32+5.76=$37.76

Speed=Distance/time

Here,

distance=15m

time=1sec

speed=15/1=15m/sec

Distance=Speed*time

time=15min=15*60sec=900sec

Distance travelled in 15 min=15*900=13,500m

=13500/1000 km=13.5Km

The wood before starting =12 feet

Left wood=6 feet

Wood used till now=12-6=6 feet

Picture frame built till now= 6/(3/4)

=8 pieces

Therefore, till now 8 pieces have been made.

Area of rectangle= L*b

here,

L=12.4 cm

b= 8.8 cm

A= L*b

= 12.4 * 8.8 = 109.12 cm^2

Let the father's age be x and son's be y

10 years before-

Father age=x-10

sons age=y-10

Given,

x-10=10(y-10)

x-10=10y-100

Given present age of father=40

therefore,

x=40

40-10=10y-100

10y-100=30

10y=130

y=130/10

y=13

Therefore present age of son=13years

Given height of gymnasium is 5/6 height of 30 foot ball

therefore height of gymnasium=5/6 * 30

=25 feet

The answer is in the image