Reggie Mobile Company has a few mobile phone service plans. Plan A offers 200 SMS and 70 minutes of talk time for $11.40. Plan B offers 450 SMS and 180 minutes of talk time for $26.10. Reggie Mobile Company is also going to launch Plan C which offers 170 SMS and 280 minutes of talk time. How much should Reggie Mobile Company charge for Plan C if the cost per SMS and the cost per minute of talk time do not vary across the three plans?
$1 = 100¢

Plan A = 200 SMS + 70 min = 1140 --- (1)

Plan B = 450 SMS + 180 min = 2610 --- (2)

Make the cost of SMS the same.

(1)_x9
1800 SMS + 630 min = 10260 --- (3)

(2)_x4
1800 SMS + 720 min = 10440 --- (4)

(4) - (3)
720 min - 630 min = 10440 - 10260
90 min = 180

1 min = 180 ÷ 90 = 2

70 min = 70 x 2 = 140

From (1):
200 SMS + 70 min = 1140 
200 SMS + 70 x 2 = 1140
200 SMS + 140 = 1140
200 SMS = 1140 - 140
200 SMS = 1000

1 SMS = 1000 ÷ 200 = 5

170 SMS
= 170 x 5
= 850

280 min
= 280 x 2
= 560

Charges for Plan C
= 170 SMS + 280 min
= 850 + 560
= 1410¢

1410¢ = 1410 ÷ 100 = $14.10

Answer(s): $14.10