A = 1/2 (A+B) solve for B

Solve for B by simplifying both sides of the equation, then isolating the variable. So in this case B=A ...

A = 1/2 (A + B)

distribute the 1/2 into the parentheses

A = 1/2A + 1/2B

Subtract 1/2A from each side

A - 1/2A = 1/2B

Combine like terms

1/2A = 1/2B

Isolate B by dividing each side by 1/2

1/2A * 2/1 = B

Multiply

2/2A = B

A = B