2.0

3.2 and -4 or 4 and-2,6 and 12

4. It takes 12 hours to make a clockwise turn (360 degrees), that is, the speed is 360 degrees/12 hours =360 degrees /( 12*60) minutes =0.5 degrees/minute.

It takes 1 hour for the minute hand to turn once (360 degrees), that is, the speed is 360 degrees/1 hour =360 degrees /60 minutes =6 degrees/minute.

The clock face (360 degrees) is divided into 12 equally, so each part (between two adjacent numbers) is 30 degrees.

So after x minutes, the angle of the hour hand is 0.5X and the angle of the minute hand is 6X.

Let's go out at 7: 00 x, because the angle between the hour hand and the minute hand on the watch is 1 10 degrees, so there is.

2 10 0.5X-6X = 1 10，

So 5.5X= 100,

So X=200/ 1 1,

So this classmate went out to buy things at 7: 20/11;

We return at 7: 00, because the angle between the hour hand and the minute hand is 1 10 degrees, so there is.

6Y-(2 10 0.5Y)= 1 10，

So 5.5Y=320,

So Y=640/ 1 1,

So this classmate went shopping at 6: 640/ 1 1 and came back.

640/ 1 1-200/ 1 1=440/ 1 1=40,

It was that classmate who spent 40 minutes out shopping.

5.3*(4-6 10)

10-4-3*(-6)

( 10-4)*3-(-6)

4-(-6)/3* 10

6.44

7.( 1) Yes, turn on 6, turn off 6 with 3 lights left, then turn on 6. (2) no.

7~. 165a 30c-45b

8.a= 1, b=- 1, and the original formula =- 1.

9.6 or 3.6