So: the ratio of number A to number B =3/4 to 2/3=3/4*(3/2)=9/8.

2. If an airplane flies for 4 seconds 1400m, then 1 min (60s) flies:15 *1400 = 21000m =2. 1 km.

Then: the time of 2 10 km is: 210/2.1=100 minutes.

3. The ratio of the number of Chinese teachers to the number of math teachers in a school is 13:7.

Suppose there is 13X Chinese teacher +03X, then there is 7X math teacher.

For example, two Chinese teachers switch to teaching mathematics: at this time, the number of Chinese teachers is: 13X-2.

Number of math teachers: 7X+2

So: (13x-2): (7x+2) = 3: 2.

2*( 13X-2)=3*(7X+2)

X=4

Now, after three-quarters of the books are lent by library A to library B, there are still 20 books left.

Then: there is 1/4 in library group A, so: 1/4 A =20.

A has 80 books.

So: B has 20 books.

Now A has 20 copies and B has 80 copies.

At present, the reading ratio of the two groups is 1: 4.