Emanuel has 745 pictures in his phone. his memory is getting full, so he starts deleting 20 pictures every day. define each variable and write the algebraic expression to describe the number of pictures left on his phone after any given number of days.

Let f be the function with variable n, which gives the number of pictures left in the phone after deleting pictures for n-days.

So f(1) gives the number of photos left after 1 day of deleting photos

     f(2) gives the number of photos left after 2 days of deleting photos

     and so on.

f(1)=745-20
f(2)=745-20-20=745-20*2
f(3)=745-20-20-20=745-20*3

so clearly f(n)=745-20n

the algebraic expression is f(n)=745-20n, 

where n is the n'th day of deleting photos, and f is the number of photos left after n days of deleting.