Exchange rate volatility
It makes more sense to use the USD as an intermediary currency.  If you know the central South African and Nigerian exchanges rates are for example 


$14.38\; ZAR = 1\; USD$ 
$364 \, NGN = 1\, USD$


then by division $\frac{14.38}{364}$ or $\frac{364}{14.38}$


$0.0395\,  ZAR \approx 1 \, NGN$
$25.3 \, NGN \approx 1 \, ZAR$