Answer:
4 months
As we show that ;
40 + (60x * 4) = 280
100 + (45x * 4) = 280
but in simultaneous equations Verizon must be set equal to 240 being 80 x 3
and T mobile must be equal to 45 x 4 = 180
so that 240+ 180 = 420 to find 4
This would be a method on distribution as "60 sign up is 1/3 more than 1st equation.
Explanation:
This is a simultaneous equation but trial and error is below to prove all is true.
step 1 make all equations same
40s + (60x) * 3 = 180x LCM = 80 x 3 = 1 1/3 of 60
100s + (45x) * 4 = 180x LCM = 45 x 4 = 1 of 45 as verizon charges 1/3 more sign up.
100s- 40s + 180x = (180x) = 240 + 180
60s + 180x = 420
s = 60
so our equations must each end with 420
when we get 60s + 180x = 420 then
420 - 180 = 240
240/60 = 4
x = 4 months
Verizon = 1st and T mobile = 2nd
40 + (60x * 5) = 340
100 + 45x * 5 = 325
with $15 out after 5 months so we try 6 months
40 + (60x * 6) = 400
100 + 45x * 6 = 370
and see this is increasing in difference, so try a smaller value of months..
We try 4 months;
40 + (60x * 4) = 280
100 + (45x * 4) = 280