How to light a tree with 6-volt bulbs if the voltage is 220V?

Suppose the resistance of the light bulb is R.
Then the current through the filament of the light bulb, according to Ohm’s law, is:
I = U / R, where I is the current through the bulb, U is the voltage applied to the bulb.

Because a 6-volt light bulb, then the normal current for it is.
I = 6 [V] / R
The formula shows that with increasing voltage, the current increases proportionally. If we increase the voltage, then the current will increase by the same amount. The light bulb may burn out.

If we want to increase the voltage from 6V to 220V, we must also increase the resistance, then the current strength will remain unchanged and the light bulb will not burn out.
The voltage increases by about 37 times
220: 6 ~ = 37
This means that the resistance should increase 37 times, then the current strength will not increase and the lamps will not burn out.
Knowing this, you can write down the formula:
I = 6 [V] / R ~ = 220 [V] / (37 * R)

Now the question is – how to increase the resistance 37 times?
It is necessary to turn on 37 bulbs in series. Because when connected in series, the resistance is added.
You can turn on sequentially and more than 37, but then the current will become less than normal and the bulbs will shine dimmer.

Answer. Turn on 37 six volt bulbs in series.