Sort:  

Blockchain forensics. Transfers to exchanges from the known steemit,inc accounts and the price of steem on those days. It's not an exact science of course hence the word "roughly", but it is a rough estimate of what they transferred and the prices on those days. Do the math. It's likely not exact, but in the proverbial ballpark.

Coin Marketplace

STEEM 0.19
TRX 0.12
JST 0.028
BTC 64839.49
ETH 3551.25
USDT 1.00
SBD 2.32