chem.mole

A. by no. of atoms
let relative abundance of lithium-6 be y, then that of lithium-7 is 1-y.
6.01y + 7.02(1-y) = 6.94
y = 7.9%
thus for a sample of lithium, there's 7.9% of lithium-6 and 92.1% of lithium-7.


B. by mass
assume that there's 1000 atoms of lithium. 79 of them are lithium-6 and 921 are lithium-7.
total mass = 79x6.01u + 921x7.02u = 6940.21u
percentage by mass = mass of specific species / total mass
thus, mass % of lithium-6 = 79x6.01u / 6940.21u
= 6.8%
and lithium-7: = 921x7.02u / 6940.21u = 93.2%