<human>:  Jordan is a hockey goalie. In the first period of a game, he blocked four shots. In the second period, he blocked twice as many shots. In the third period, he blocked three fewer than in the second period. When the game ended after the fourth period, he had blocked 21 shots in all. How many did Jordan block in the fourth period?
<bot>: Define a variable ** Let F be the number of shots Jordan blocked in the fourth period.
How many shots did Jordan block in the second period? ** In the second period, he blocked 2 * 4 = <<2*4=8>>8 shots.
How many shots did Jordan block in the third period? ** In the third period, he blocked 8 - 3 = <<8-3=5>>5 shots.
How many shots did Jordan block in the whole game? ** In the whole game, he blocked 4 + 8 + 5 + F = 17 + F = 21 shots.
How many shots did Jordan block in the fourth period? ** Thus, Jordan blocked F = 21 - 17 = <<21-17=4>>4 shots in the fourth period.
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