John Mobile Company has a few mobile phone service plans. Plan A offers 250 SMS and 60 minutes of talk time for $24.30. Plan B offers 450 SMS and 110 minutes of talk time for $43.80. John Mobile Company is also going to launch Plan C which offers 190 SMS and 260 minutes of talk time. How much should John 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 + 60 min = 2430 --- (1)

Plan B = 450 SMS + 110 min = 4380 --- (2)

Make the cost of SMS the same.

(1)_x9
2250 SMS + 540 min = 21870 --- (3)

(2)_x5
2250 SMS + 550 min = 21900 --- (4)

(4) - (3)
550 min - 540 min = 21900 - 21870
10 min = 30

1 min = 30 ÷ 10 = 3

60 min = 60 x 3 = 180

From (1):
250 SMS + 60 min = 2430 
250 SMS + 60 x 3 = 2430
250 SMS + 180 = 2430
250 SMS = 2430 - 180
250 SMS = 2250

1 SMS = 2250 ÷ 250 = 9

190 SMS
= 190 x 9
= 1710

260 min
= 260 x 3
= 780

Charges for Plan C
= 190 SMS + 260 min
= 1710 + 780
= 2490¢

2490¢ = 2490 ÷ 100 = $24.90

Answer(s): $24.90