He paid $240 for a dozen tires or 12 tires

For 1 tire he paid: 240/12 = $20

A tire dealer sells 2 tires for as much as he paid for 3 tires and he paid 3*20 = $60 for 3 tires

Thus the profit for 2 tires is: 60 - 2*20 = 60 - 40 = $20

The profit for 1 tire will be: 20/2 = $10

For 12 tires the total profit will be: 12*10 = $120.