How ids the value of 4 in 864 related to the value of 4 in 846?

Mat bought a phone for $100. he has to pay $30 per mouth. if he has paid $640 in total, how many mouth has he had the phone?

X/y + 4y/x divided by x/y - 2y/x explain as in depth as you can how you simplfied the fractions. i don't know where to start on this : /

Question 6(multiple choice worth 5 points) (05.02 lc) what is the value of x? a right angle is shown divided into two parts. the measure of one part of the right angle is 10 degrees. the measure of the other part is 2x 20 40 45 85