$0.9=0.4+B+0.4 \times B$

Step 1 ：Combine like terms on the right side of the equation to get \(1.4B + 0.4 = 0.9\).

Step 2 ：Subtract 0.4 from both sides of the equation to get \(1.4B = 0.5\).

Step 3 ：Divide both sides of the equation by 1.4 to solve for B, \(B = 0.357142857142857\).

Step 4 ：Final Answer: \(B = \boxed{0.357142857142857}\)