Answer
13.7 million metric tons of lithium.
Work Step by Step
Molecular weight of lithium carbonate: $2\times 6.94+12.011+3\times 15.999=73.89\ g/mol$
Mass fraction of lithium:
$2\times 6.94/73.89\times 100\%=18.8\%$
From 73 million metric tons of lithium carbonate: $73\times 18.8/100 = 13.7$ million metric tons of lithium.
