Answer:
c = 160 - 3(45)
Explanation:
if c = the number of characters left;
160 = maximum number of characters that can be used in a text
3*45 = number of characters in sentences already used. (There are 3 sentences of 45 characters.)
(# of char left) = (max # char) - (# char already used) so:
c = 160 - 3*(45)
c = 160 - 135
c = 25