(-3a4b3 )(-5a4b5)
Multiply like variables, multiply the coefficients, remembering that multiplying two negative numbers give a positive result.
Multiplying variables in exponential form, we add the exponents
Result (-3)(-5) =15
a4 x a4 = a8
b3 x b5 =...

Let cost of each book be x and cost of backpack =y
2x + y =26 (i)
Each book cost $8 less than a backpack
x=y-8 (ii)
Substitute for x in equation (i)
2(y-8) +y = 26
2y - 16 +y =26
3y = 42
y =14 (Cost of backpack)
x = cost of book =...

Because the compound is an oxide it contains only nitrogen and oxygen.
The % oxygen in the compound is 100-25.9% =74.1%
Remember in empirical formula we can treat the % as having 100g of the compound, there would be 25,9 g nitrogen and 74.1 g oxygen.
Steps in empirical formula
1...

The total sales figure is =107%. Your sales before tax is =100%. Use the ratio relationship to fine the sales before tax.
6645.16/107 = x(sales before tax)/100
Solve for x gives you 6210.43!
Check you answer 7% of 6210.43 = 434.73.
Total with tax = 6645.16...

Rewrite fractions with the same denominator (preferably the lowest factor for both numbers. This is 18 in his case.
-5/6 + (-5/18) = -15/18 + (-5/18)
Now that we have done that we can add the numerators and place the answer over the denominator.
-15 + (-5) = -20/18
The...