Solve for A
A=\frac{1991B}{1990}
B\neq 0
Solve for B
B=\frac{1990A}{1991}
A\neq 0
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1990A=1991B
Multiply both sides of the equation by 1990B, the least common multiple of B,1990.
\frac{1990A}{1990}=\frac{1991B}{1990}
Divide both sides by 1990.
A=\frac{1991B}{1990}
Dividing by 1990 undoes the multiplication by 1990.
1990A=1991B
Variable B cannot be equal to 0 since division by zero is not defined. Multiply both sides of the equation by 1990B, the least common multiple of B,1990.
1991B=1990A
Swap sides so that all variable terms are on the left hand side.
\frac{1991B}{1991}=\frac{1990A}{1991}
Divide both sides by 1991.
B=\frac{1990A}{1991}
Dividing by 1991 undoes the multiplication by 1991.
B=\frac{1990A}{1991}\text{, }B\neq 0
Variable B cannot be equal to 0.
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