So a unit of steel sells for $79, the steel mill produces half a unit per second. The revenue of a steel mill is thus $79/2 = $39.5. However, the steel mill takes a unit of iron per second as input, so it uses $25 per second to keep functioning. $39.5-$25 is $14.5 net revenue. However it's likely that iron price is like $25.2 or something causing it to be rounded down to $14 rather than rounded up to $15.