Question 123293
Let the lenngth of the call = N minutes
Cost with Card type "A"=30cents+2N cents=30+2N cents
Cost with Card type "B"=10cents+6N cents=10+6N cents
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Now equate both of the values 
10+6N=30+2N
6N-2N=30-10
4N=20
N=20/4=5 minutes
For a call lasting 5 minutes the cost via both the cards is one and the same.
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Verification:

Type A card cost for 5 min=30cents+2N =30+(2*5)=30+10=40 cents
Type B card cost for 5 min=10cents+6N =10+(6*5)=10+30=40 cents

okay
good day