The idea is to try and make the number of SMS for Plan A and Plan B the same for direct comparison.

Plan A offers 300 SMS and 70 min for $25.50.

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This means that

Plan A offers 600 SMS and 140 min for $51.
Plan A offers 900 SMS and 210 min for $76.50.
Plan A offers 1200 SMS and 280 min for $102.
And so on.

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Plan B offers 400 SMS and 100 min for $35,

This means hat

Plan B offers 800 SMS and 200 min for $70.
Plan B offers 1200 SMS and 300 min for $105.
And so on.

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Do you see two SMS numbers which are the same? Let's pull them out here.

Plan B offers 1200 SMS and 300 min for $105.
Plan A offers 1200 SMS and 280 min for $102.

Their difference lies in the extra 20 min offered by Plan B for that extra $3.

So, 20 min of talktime must cost $3.

1 min of talktime must cost $3 divided by 20 = $0.15.

300 minutes of talktime must cost $0.15 * 300 = $45.

This means that

1200 SMS must cost $105 - $45 = $60.
1 SMS must cost $60 divided by 1200 = $0.05.

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Cost of 200 SMS
= $0.05 * 200
= $10

Cost of 140 minutes of talktime
= $0.15 * 140
= $21

Cost of Plan C
= Cost of 200 SMS + Cost of 140 min talktime
= $10 + $21
= $31