How much should I give you back? 9 dollars.

You have +9.

-21 + 30 = would be the same as

+30 + (-21) =

30 + 1(-21) = (remember a (-) times a (+) = (-))

30 – 21 = 9

You can do it on a regular calculator…..just enter a negative sign,21,+30=9

this can be rewritten as: 30 – 21

In addition, when you have different signs (positive and negative) … you find the difference (subtract). If the signs are the same (+,+ or -,-) you add the two numbers.

In this case, you should subtract the two numbers and the ‘sign’ on the answer should be the same as the larger number.

If it had been: (+21) + (+30) = +51 [same as 21+30=51]

You would add the two numbers and it would have the same sign as the numbers in the problem…positive.

and

If it had been: (-21) + (-30) = -51 [same as -(21+30)= -(51) = -51]

You would add the two numbers and it would have the same sign as the numbers in the problem…negative.

Hope this helps…been awhile since I’ve had math.

You need to be able to understand your basic math. You will need it not just for school but also for calculations you will need to make in life.

30 – 21 = ?. You probably know the answer to that.

But let us say you had a similar problem

21 – 30 = ?

You might think of the problem as

30 – 21 = -?

Hope that helps. BTW, a calculator should only be used once you understand the math the calculator is doing for you. Sometimes the thinking used for math is used in other places in life where numbers aren’t involved.

try it this way do 30+ -21… you could drop the + and just make it 30-21 and you get 9

your awnser is incorrect, the – should be positive.