Question 471335
during the calender year her basic monthly telephone rate increased from $17.10 to $18.40. 
If she paid a total of $207.80 for basic telephone service for the calender year, in what month did the rate increase take effect
:
Let m = no. months she paid $17.10
then
(12-m) = no. of months she paid $18.40
:
17.10m + 18.40(12-m) = 207.80
17.10m + 220.80 - 18.40m = 207.80
17.10m - 18.40m = 207.80 - 220.80
-1.30m = -13
m = {{{(-13)/(-1.3)}}}
m = +10 months at the lower rate, then the increase took effect in the 11th month (Nov)
:
:
see if that's true
10(17.10) + 2(18.40) = 
171.00 + 36.80 = 207.80