PART A: $60d + $0.65(600) = $3,390
PART B: $60d + $0.65(600) = $3,390
Multiply $0.65*600
$60d + $390 = $3,390
Subtract 390 from both sides
$60d + $390-$390 = $3,390 - $390
$60d = $3,000
Divide both sides by $60 to figure out d
$60d/$60 = $3,000/$60
d = 50
Part C
50 days