If you bought all $100k (now $120k) at the same time, then yes, selling $60k would result in a $10k capital gain. This would also be true if you used average cost basis tracking whether or not you bought all at the same time, but it's better to use specific identification tracking as you can then choose what lots to sell if some have different amounts of gains than others.
How much you'll pay in tax will depend on your filing status and other income, but will be between 0 and 20%, plus I think 3.8% medicare surcharge if you're income is especially high.