Perry Mobile Company has a few mobile phone service plans. Plan A offers 250 SMS and 70 minutes of talk time for $23.90. Plan B offers 300 SMS and 180 minutes of talk time for $30.60. Perry Mobile Company is also going to launch Plan C which offers 150 SMS and 270 minutes of talk time. How much should Perry 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 = 250 SMS + 70 min = 2390 --- (1)

Plan B = 300 SMS + 180 min = 3060 --- (2)

Make the cost of SMS the same.

(1)_x6
1500 SMS + 420 min = 14340 --- (3)

(2)_x5
1500 SMS + 900 min = 15300 --- (4)

(4) - (3)
900 min - 420 min = 15300 - 14340
480 min = 960

1 min = 960 ÷ 480 = 2

70 min = 70 x 2 = 140

From (1):
250 SMS + 70 min = 2390 
250 SMS + 70 x 2 = 2390
250 SMS + 140 = 2390
250 SMS = 2390 - 140
250 SMS = 2250

1 SMS = 2250 ÷ 250 = 9

150 SMS
= 150 x 9
= 1350

270 min
= 270 x 2
= 540

Charges for Plan C
= 150 SMS + 270 min
= 1350 + 540
= 1890¢

1890¢ = 1890 ÷ 100 = $18.90

Answer(s): $18.90