The table below summarises the plug types, voltage and frequency of electrical current in both Belarus and Cook Islands.
|Frequency of electrical current||50Hz||50Hz|
Belarus uses Plug Type C.
You will need this adaptor:
Socket Type C
(Belarus) to Plug Type I
The voltages found in Belarus (220V) and Cook Islands (240V) are not exactly the same, but the difference is tolerable, so you should not need a transformer.
The frequency found in Belarus and Cook Islands is exactly the same (50Hz). Timing devices and appliances that use motors will function as expected under this frequency.