The first question:

120/ 10x 0.4+(- 174.8)

= 12x0.4- 174.8

=4.8- 174.8

=- 170 (m)

A: The altitude of point B is-170 meters.

The second question:

Height difference between point C and point A =-68.8-(-174.8) =174.8-68.8 =106 (m).

So the time from A to C is: 30x106/10 = 318 (seconds).

A: The time from point A to point C is 3 18 seconds.

This topic mainly investigates the mixed operation of rational numbers and the mixed operation of positive numbers and negative numbers.

Negative addition and subtraction formula:

1, the same name points, different names benefit.

Divide by the same name, that is, when two numbers with the same symbol are subtracted, the symbol of the minuend is in front of brackets, and the absolute value of the minuend minus the absolute value of the minuend is in brackets.

2. Nothing is negative and nothing is positive.

When two numbers with different names are subtracted, the symbol of the minuend is in front of the bracket, and the absolute value of the minuend plus the absolute value of the minuend is in the bracket.

3. No progress is negative, no progress is positive, that is, 0 is negative and 0 is positive. Negative number 1+ negative number 2=- (negative number 1+ negative number 2)= negative number.

Negative number+positive number = the sign takes the sign of addend with larger absolute value, and the numerical value takes the value of "larger absolute value minus smaller absolute value".

Extended data:

Negative elementary arithmetic order:

1, same level, from left to right;

2. Two-stage operation, first multiply and then divide, then add and subtract. ?

3. When there are brackets, count the inside of brackets first, and then count the outside of brackets;

4. When there are multiple brackets, count the brackets first, then the brackets inside, then the braces inside, and finally the brackets outside.