Question 419224
Let s = Don's salary
Let p = Don's commission rate

Note that the dollar amount of Don's commission is a percent of the sales, so Don's dollar commission on sales of $4000 is 4000p and Don's dollar commission on sales of $6000 is 6000p.  His salary is fixed, no matter how much he sells.  So, using his compensation totals for the two weeks, we have:

In week 1:  s + 4000p = 600
In week 2:  s + 6000p = 740

Solving these two simultaneous equations, we have equation 1:
s = 600 - 4000p

Substituting into equations 2, we have

600 - 4000p + 6000p = 740
600 + 2000p = 740   (combining like terms)
2000p = 140   (subtracting 600 from both sides)
p = 0.07  (dividing both sides by 2000)

This means that Don's commission is 0.07 of sales, or 7%.

In figuring up Don's weekly salary, we can use either of the two equations, say equation 1:

s + 4000p = 600   (equation 1)
s + 4000(0.07) = 600  (substituting value for variable p)
s + 280 = 600   (simplifying the term)
s = 320   (subtracting 280 from both sides)

This Don's weekly salary is $320 and his commission rate is 7%