Nitrogen-13 has a half-life of 10 minutes. Given a starting mass of 2.00 g of nitrogen-13, how many grams will remain at the end of 30 minutes?

k = 0.693/t1/2

ln(No/N) = kt
No = 2.00
N = unknown
k from above
t = 30 min
Solve for N.

You can also use this.
30 min is obviously 3 half lives; therefore, 23 = 8 and 2.00 x (1/8) = ?