kazoo

Accomplished tomb raider Iowa Kowalski approaches a bridge with a sign stating the maximum weight bearable is 80kg. He estimates his own weight to be 78kg, and knows the weight of the 4 gold ingots in his sack to be approximately 1kg each. After a moment of consideration, he safely crosses the bridge.

How did he do so?

__________________
There ain't nothin' more powerful than the odor of mendacity -Big Daddy