w/5=7/10 solve for w
In a proportion written: $\displaystyle w:5 :: 7:10$, the product of the extremes (the 1st and 4th terms) = the product of the means (the 2nd and 3rd terms). Knowing this, then
$\displaystyle \frac{w}{5}=\frac{7}{10}$
$\displaystyle 10w=35$
$\displaystyle w=\frac{35}{10}=\frac{7}{2}$
Try to get all variables (letters) on one side, numbers on the other. Your problem is that the answer, w, is divided by 5 on the lhs. You must do the opposite (ie multiply by 5) throughout the expression (do it on both sides) to remove the offending 5 and hence get w = ....
So
$\displaystyle \frac{w}{5}= \frac{7}{10}$
Multiply both sides by 5
$\displaystyle 5\times\frac{w}{5}= 5\times\frac{7}{10}$
Do some cancellation on lhs and tidying up on rhs
$\displaystyle w=\frac{7}{2} = 3.5$.
Hope that helps.