First write out the expressions for each of the two carriers:

Carrier A is $1.00 for the first 20 minutes and $0.07 for every minute after 20, so that expression for the total cost would be: 1+0.07(t-20). (The "t-20" is how you write the phrase "numbers of minutes past 20.")

Carrier B is simply $0.06 per minute, so that expression would be: 0.06t.

The question asks for when the costs would be the same, so set the expressions equal to each other: 1+0.07(t-20) = 0.06t. Distribute to get 1+0.07t-1.4 = 0.06t, and combine like terms to get: 0.01t = 0.4. Solve that to get t = 40.

## 11/23/09

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