The formula for the area of a triangle is a=1/2bh, where b represents the length of the base and h represents the height. a. solve the formula a=1/2bh for b.

 b = "  " .
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Explanation:
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A = (1/2) * b * h = (b * h) / 2 ; 

→  A = (b * h) / 2 ; 

Rearrange the formula / equation ; and solve in terms of "b" ; 

Multiply each side of the equation by "2" ; 

2 * A = 2* { (b * h) / 2 } ;

to get:

 2 A  = (b * h) ; 

↔  (b * h) = 2 A ; 

Divide each side of the equation by "h" ;  
 to isolate "b" on one side of the equation; & to solve "in terms of, "b" " ;  

 → (b * h) / h = (2 A) / h ;

to get:
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→ b = " " .
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The key is the word about, they want an estimation.63 is closer to 60, 71 is closer to 7060 + 70 = 130ryan counted about 130 birds