what is the magnitude of total voltage in the below circuit?
Whenever you have a resistor in series with an (ideal) inductor, if the current is sinusoidal, their voltages will be 90º apart. The total voltage \$V_T\$ is the vector sum of \$V_R\$ and \$V_L\$. Since \$V_R\$ and \$V_L\$ are orthogonal (due to the 90º phase difference), the module of \$V_T\$ can be easily computed as \$|V_T|=\sqrt{|V_R|^2+|V_L|^2}=\sqrt{4^2+3^2}=\sqrt{25}=5\$.

The reason for those 90º is: if \$I=\sin(wt)\$, then it will be \$V_R=R\sin(wt)\$ and \$V_L=L\dfrac{dI}{dt}=L·w\cos(wt)\$ .

Update: stevenh is right, and my explanation could be confusing. I was indeed assuming, from the OP, some two-dimensional interpretation of sinusoidal voltages and currents (where A·sin(wt+phi) is just a rotating vector with radius A and phase wt+phi), although complex numbers are not really required.