42+21-x=57
\dfrac{x}{x + 15} = \dfrac{4}{9}
4b+5=1+5b
9g=3(-4+5g)
make a the subject of the formula:
3b = 10c - 7a / 4
\dfrac{5x + 1}{3x - 4} = \dfrac{16}{5}
m=-(4+m)+2
-7+4m+10=15-2m
5 2/6 + 6 1/3
\dfrac{3 - 2x}{1 - 2x} = \dfrac{3}{5}
