The value of alternating e.m.f. (E) in the given circuit is

A series LCR circuit has the voltage across the inductor as $V_L = 20\text{ V}$, across the capacitor as $V_C = 50\text{ V}$, and across the resistor as $V_R = 40\text{ V}$.

The resistor voltage is in phase with the current, while the inductor voltage leads by $90^\circ$ and the capacitor voltage lags by $90^\circ$. Thus $V_L$ and $V_C$ differ in phase by $180^\circ$.

The alternating e.m.f. is the phasor sum:
$E = \sqrt{V_R^2 + (V_L - V_C)^2}$

Substituting the values:
$E = \sqrt{(40)^2 + (20 - 50)^2} = \sqrt{1600 + 900} = \sqrt{2500} = 50\text{ V}$

Final answer: $\boxed{50\text{ V}}$