Dylan Mobile Company has a few mobile phone service plans. Plan A offers 200 SMS and 90 minutes of talk time for $14.70. Plan B offers 300 SMS and 170 minutes of talk time for $23.10. Dylan Mobile Company is also going to launch Plan C which offers 150 SMS and 280 minutes of talk time. How much should Dylan 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 + 90 min = 1470 --- (1)

Plan B = 300 SMS + 170 min = 2310 --- (2)

Make the cost of SMS the same.

(1)_x3
600 SMS + 270 min = 4410 --- (3)

(2)_x2
600 SMS + 340 min = 4620 --- (4)

(4) - (3)
340 min - 270 min = 4620 - 4410
70 min = 210

1 min = 210 ÷ 70 = 3

90 min = 90 x 3 = 270

From (1):
200 SMS + 90 min = 1470 
200 SMS + 90 x 3 = 1470
200 SMS + 270 = 1470
200 SMS = 1470 - 270
200 SMS = 1200

1 SMS = 1200 ÷ 200 = 6

150 SMS
= 150 x 6
= 900

280 min
= 280 x 3
= 840

Charges for Plan C
= 150 SMS + 280 min
= 900 + 840
= 1740¢

1740¢ = 1740 ÷ 100 = $17.40

Answer(s): $17.40