552
Here's how I did it:
I set up my two equations:
xy=540
x+y=12
Then, by subtraction
x=12-y
By substitution:
xy=540
(12-y)y=540
12y-y2=540
-y2+12y-540=0
Through the quadratic formula...
y=6 ± -6i√14
If you use addition for y, then you should subtract for x, so that x + y = 12
16.5
1660
6
28
One trick for multiplying by 12 is to multiply the number by 10 and then add two times the number. For example, if you want to multiply 7 by 12, first multiply 7 by 10 to get 70, and then add 7*2=14 to get 84.
552
702
16.5
1660
-121
71
1.52
6
Multiply the three-digit number by the one's digit, or last digit, of the two-digit number. That is your first part. Now multiply by the second-to-last digit, or ten's digit, and multiply the result by 10. That is your second part. Add the two parts and that is your answer.
32
7
28