At a certain company, 64.5% of the employees have engineering degrees.
To express the percentage as a decimal we need to remove the % symbol.
To remove % symbol we always divide by 100
When we divide by 100 we move the decimal point two places to the left.

The decimal vale of 64.5% is 0.645
It is quite simple actually since it is simple division with the negative rules.
Just simplify the problem and use the negatives to determine if it is positive or negative. (two negatives equal a positive and one negative equals a negative)
X = -6y - 12
4x + 5y =-39
4x + 5y = -39
-4(-6y - 12) + 5y = -39
-4(-6y) + 4(12) + 5y = -39
24y + 48 + 5y = -39
29y + 48 = -39
29y = -87
y = -3
x = -6y - 12
x = -6(-3) - 12
x = 28 - 12
x = 6
(x, y) = (6, -3)
One method is to round 997 up to 1000, multiply by 8, and then subtract 8 times 3. This would give you the solution of 7976.
Another method (Which I personally wouldn't use) is to recursively double 997. This is more difficult, although effective. After you double 997, double the resulting number, and then double the resulting number from that, you have the solution. This is because 2^3 is 8.