Counter A holds 20% of the stock held at counter B, and counter B holds 40% of the stock held at counter C. Counter C's stock is what per cent of counter A's?

$A=0.20B$ and $B=0.40C$, so $A=0.20\times 0.40xC=0.08C$. Turning that round, $C=A/0.08=12.5A=1250$%. Numbers help: if $C=1,000$ then $B=400$ and $A=80$, and 1000/80 = 12.5. 8% is A as a share of $C$ - the comparison the other way round, and the answer to a different question. 750% adds the two reversals (1/0.2 = 500% and 1/0.4 = 250%) instead of multiplying them. 125% is a slipped decimal.