Solution: Let the principal be X yuan.

4*2.7%x+x=5540

1. 108x=5540

x=5000

A: The principal is 5000 yuan.

2.

Solution: Let the interest rate in 2000 be X, and the interest rate in 1000 be 3.24%-X.

2000 x( 1-20%)+ 1000(3.24%-x)( 1-20%)= 43.92

1600x+25.92-800x=43.92

800x= 18

x=0.0225=2.25%

3.24%-2.25%=0.99%

A: The annual interest rates of the two kinds of savings are 2.25% and 0.99% respectively.

4.

After 3 years, you can get:

50000×2.89%×3+50000=54335 yuan

5.

Solution: set up a x people and b 150-x people.

600 x+ 1000( 150-x)= 100000

600 x+ 150000- 1000 x = 100000

400x=50000

x= 125

150- 125=25

A: A recruit 125 people, and B recruit 25 people.

6.

Solution: Last year, the key class recruited X students, and the ordinary class recruited 1 100-x students.

x * 10%+( 1 100-x)20% = 200

0. 1x+220-0.2x=200

0. 1x=20

x=200

Key classes this year: 200×( 1+ 10%)=220 students.

This year, 220 students registered in key classes.

7.

Solution: Let the insured amount be X yuan.

( 16000-9600)/ 16000 * x = 6000

0.4x=6000

x= 15000

Answer: the insured amount 15000 yuan.