Really.
You have to multiply each part of equation using the same factor
your equation : ry=10*zoom*(1/y)
step 1 : Factor = *y
ry *y=10*zoom*(y/y)
( y/y=1 ) => ry*y=10*zoom
step 2 factor= 1/ry
y*ry /ry = 10*zoom /ry
simplify the quation. => y= 10*zoom/ry
You have to multiply each part of equation using the same factor
your equation : ry=10*zoom*(1/y)
step 1 : Factor = *y
ry *y=10*zoom*(y/y)
( y/y=1 ) => ry*y=10*zoom
step 2 factor= 1/ry
y*ry /ry = 10*zoom /ry
simplify the quation. => y= 10*zoom/ry