12 31/100 is the mixed fraction because in a decimal all the numbers to the left are whole numbers and all the numbers to the right are numbers that are part of a whole. Just like in a mixed fraction there is a whole number and a fraction in a mixed number. Since 12 is in the left it is the whole number and since 31 is to the right and in the place value 31 is ending in the hundredths place so that's why the fraction is, 31/100. That's how you came to know that 12 31/100 is the fraction for 12.31.
For this one, we need to make an equation to make it look easier
(0.10 × m) + 500 = 1280
We solve for m
Subtract 500 from both sides and we get
0.10 × m = 780
Now, divide both sides by 0.10
And m would be equal to 7800
Let's see if this is correct.
Five hundred dollars more than 10% of m
We found that 10% of m is 780, now we add 500 to make the statement true. It will give us a total of 1280, which is equal of what the question says.
I hope this helps.
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No. AB is not perpendicular to AC