Jeremy Mobile Company has a few mobile phone service plans. Plan A offers 200 SMS and 60 minutes of talk time for $14.40. Plan B offers 300 SMS and 130 minutes of talk time for $23.20. Jeremy Mobile Company is also going to launch Plan C which offers 190 SMS and 270 minutes of talk time. How much should Jeremy 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 + 60 min = 1440 --- (1)

Plan B = 300 SMS + 130 min = 2320 --- (2)

Make the cost of SMS the same.

(1)_x3
600 SMS + 180 min = 4320 --- (3)

(2)_x2
600 SMS + 260 min = 4640 --- (4)

(4) - (3)
260 min - 180 min = 4640 - 4320
80 min = 320

1 min = 320 ÷ 80 = 4

60 min = 60 x 4 = 240

From (1):
200 SMS + 60 min = 1440 
200 SMS + 60 x 4 = 1440
200 SMS + 240 = 1440
200 SMS = 1440 - 240
200 SMS = 1200

1 SMS = 1200 ÷ 200 = 6

190 SMS
= 190 x 6
= 1140

270 min
= 270 x 4
= 1080

Charges for Plan C
= 190 SMS + 270 min
= 1140 + 1080
= 2220¢

2220¢ = 2220 ÷ 100 = $22.20

Answer(s): $22.20