If a car has a depreciation rate of 20%, what is the value of the car after 6 years of its?

original value is $35,000?

Value is $0 after 6 years. The $35,000 has been entirely depreciated. These may not be the exact figures for each year but since you say a 20% rate it will have a 5 year expected life which actually depreciates over 6 years.
1st year 100% - 10% = 90% $31,500
2nd " 90% - 20% = 70% $24,500
3rd " 70% - 20% = 50% $17,500
4th " 50% -20% = 30% $10,500
5th " 30% - 20% = 10% $ 3,500
6th " 10% - 10% = 0% $ 0

This is for writing off the value for tax purposes only and doesn't reflect the true value of the car at the end of 6 years

35 000 * (0,80^6) = 9 175,04

If this is for taxes, they may have a depreciation schedule, ie you take X percent in year Y.
Straight line 20% depreciation, the car would be worth scrap value at the end of year 5.

Depends upon how you calculate the depreciation.

If it's straight line you can only depreciate it for 5 years -- 20% x 5 = 100%. At the end of 5 years you'd be at salvage value and could not take any further depreciation. So, assume that salvage value was 10% of the acquisition cost you'd depreciate $31,500 and take $6,300 in depreciation each year for 5 years. At the end of year 5 the book value would be $3,500 and no further depreciation would be possible so the book value at the end of year 6 would remain at $3,500.

If you're using the declining balance method you apply the 20% rate to the remaining value each year so the amount of depreciation will decline each year since it's based on a lower value each year. The first year depreciation would be $7,000 (20% of $35,000) and the second year would be $5,600 (20% of $28,000) and so forth. I'll let you crunch the numbers from there. Hint: Daan D's answer is correct for the declining balance method, it's just not in the common US format but $9,175.04 is correct.