In the Singapore Primary School math system, a lot of emphasis is placed on model drawing without explicitly using algebra. It's possible to solve your first problem this way.
Initial ratio $= 3:5$ Total number of units = $3+5 = 8$
An internal transfer of $5$ objects now occurs.
Final ratio $=7:9$. Total number of units = $7+9 = 16$
Since there has been no external transfer, you can compare the two by getting the total number of units to be the same. The initial ratio is multiplied by two to give $6:10$, which has a total of $16$ units, equal to the final ratio.
Clearly, $1$ unit has been transferred between the two sides of the ratio to go from the initial to the final, and that's equivalent to $5$ marbles.
So the total of $16$ units is equivalent to $(16)(5)=80$ marbles. [answer]
For your second problem, if $5$ is equivalent to $17\%$, then $1\%$ is $\frac{5}{17}$. You can immediately get the total ($100\%$) from this by multiplying by $100$, so the total is $\frac{(5)(100)}{17} \approx 29.41$. But if you really wanted $83 \%$, it is $\frac{(5)(83)}{17} \approx 24.41$