Capital gain on shares

Last year I had 25 shares valued each at 100 so total value=2500.

I sold 15 out of these at 1800.

So should I,

1.value the remaining 10 shares at 700(2500-1800). or

2.value the remaining 10 shares at 1000(2500-1500[cost of 15 shares) and calculate capital gain?\

capital gain is .........1800-1500 = 300

Please answer and explain whether to compute capital gain or not as the all shares are not sold?

And at which value I should record?