Daniel Mobile Company has a few mobile phone service plans. Plan A offers 200 SMS and 70 minutes of talk time for $16.80. Plan B offers 300 SMS and 130 minutes of talk time for $26.20. Daniel Mobile Company is also going to launch Plan C which offers 190 SMS and 270 minutes of talk time. How much should Daniel 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 = 1680 --- (1)

Plan B = 300 SMS + 130 min = 2620 --- (2)

Make the cost of SMS the same.

(1)_x3
600 SMS + 210 min = 5040 --- (3)

(2)_x2
600 SMS + 260 min = 5240 --- (4)

(4) - (3)
260 min - 210 min = 5240 - 5040
50 min = 200

1 min = 200 ÷ 50 = 4

70 min = 70 x 4 = 280

From (1):
200 SMS + 70 min = 1680 
200 SMS + 70 x 4 = 1680
200 SMS + 280 = 1680
200 SMS = 1680 - 280
200 SMS = 1400

1 SMS = 1400 ÷ 200 = 7

190 SMS
= 190 x 7
= 1330

270 min
= 270 x 4
= 1080

Charges for Plan C
= 190 SMS + 270 min
= 1330 + 1080
= 2410¢

2410¢ = 2410 ÷ 100 = $24.10

Answer(s): $24.10