In both the cases the heat supplied by the electric kettle will be same. Also since the same kettle is used, the resistance(R) will be same in both the cases.
Let, V1= 220 v , t1 = 5 min and V2 = 200 v, t2 = ?
using the formula below, the required time can be calculated.
(V12t1)/R = (V22 t2)/ R