Question

As a salesperson at Roaring Waves Beach Supplies, Carissa receives a monthly base pay plus commission on all that she sells. If she sells $500 worth of merchandise in one month, she is paid $405. If she sells $900 of merchandise in one month, she is paid $489. Find Carissa's salary when she sells $1,700 worth of merchandise.

Answer 1

y = her salary,
m = rate of commission,
x = amount of merchandise sold 
b = monthly base pay 

 sets of data to calculate the formula
(500, 405) and (900, 489)

use the slope formula to find m:
(489 - 405) / (900 - 500)
m= 84 / 400 = 0.21  (21% commission) 

find the base pay by substituting known values for x,y, and m, then solve for b: 
 405 = (0.21)(500) + b 
 405 = 105 + b
b = 300

now we know the equation for her salary is:
y = 0.21x + 300

To find the salary when she sells $1,700 worth of merchandise, put 1700 for x and solve for y:
y = 0.21(1700) + 300 = 657