Answer:
after 10 months
Step-by-step explanation:
Let x be the number of months and y be the amount they still owe.
Sin Ian borrows $1000 from his parents, then the y-intercept b= 1000 since he owes $1000 when x = 0. He pays them back $60 each month The slope is then m = -60 . Substituting in b = 1000 and m = -60 into the slope-intercept form of a line then gives y= mx + b=-60x +1000.
Sin Ken borrows $600 from his parents, then the y-intercept b = 600 since he owes $600 When x= 0. He pays them back $20 per month so the amount he owes decreases $20 each month. The slope is then m = -20 . Substituting in
b= 600 and m = -20 into the slope-intercept form then gives y = mx +b = -20x + 600.
They will owe the same amount when they have the same y-coordinate. Therefore -60x+ 1000= y= -20x+600. Solve this equation for x:
-60x+ 1000 = -20x+ 600
1000 = 40x+ 600
400 = 40x
10=x
They will then owe the same amount after 10 months.
