Seth Mobile Company has a few mobile phone service plans. Plan A offers 150 SMS and 70 minutes of talk time for $14.80. Plan B offers 240 SMS and 160 minutes of talk time for $25.60. Seth Mobile Company is also going to launch Plan C which offers 170 SMS and 200 minutes of talk time. How much should Seth 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 = 150 SMS + 70 min = 1480 --- (1)

Plan B = 240 SMS + 160 min = 2560 --- (2)

Make the cost of SMS the same.

(1)_x8
1200 SMS + 560 min = 11840 --- (3)

(2)_x5
1200 SMS + 800 min = 12800 --- (4)

(4) - (3)
800 min - 560 min = 12800 - 11840
240 min = 960

1 min = 960 ÷ 240 = 4

70 min = 70 x 4 = 280

From (1):
150 SMS + 70 min = 1480 
150 SMS + 70 x 4 = 1480
150 SMS + 280 = 1480
150 SMS = 1480 - 280
150 SMS = 1200

1 SMS = 1200 ÷ 150 = 8

170 SMS
= 170 x 8
= 1360

200 min
= 200 x 4
= 800

Charges for Plan C
= 170 SMS + 200 min
= 1360 + 800
= 2160¢

2160¢ = 2160 ÷ 100 = $21.60

Answer(s): $21.60