數學問題，請幫我

1. .Ann has an Octopus card with an amount of $13.She buys some fast food wich costs$22 .Then she re-loads the card with $100.What is the amount on the card now?

Balance of her Octpus card after buying fast food = $13 - $22 = -$9
Aftering re-loading $100, the Octopus balance = -$9 + $100 = $91
The answer is $91.


2.Do you know what is the application of directed numbers?
In the above exercises, we used directed numbers to represent temperature below or above 0^oC.Can you give two more examples of the application of directed numbers? Write down what positive numbers and negative numbers respectively.

Many of the numbers we use represent situations which have directions as well as size
The numbers which have a direction and a size are called directed numbers.
Once a direction is chosen as positive (+), the opposite direction is taken as negative (- )

Some other examples of application of directed numbers are,
a) Profit, postive profits means earning; negative profit means loss; e.g. +$100 means an earning of $100, -$100 means a loss of $100.
b) slope, positve slope means up slope, negative slope means down slope; e.g., +30º means 30º upslope, -30º means 30º downslope.
c) one more example is the floor number in a building, we usually denote floors above Ground Floor at 1/F, 2/F, 3/F and etc, and the floors under Ground Floor as B1/F (or -1/F), B2/F (-2/F). This is also another example application of directional numbers.