Let's just start that first off all, it was a rhetorical figure, seven being a significant, magical number - "If Cain is avenged seven times, then Lamech seventy-seven times", for instance. You're not supposed to start counting when Lincoln says "four scores and seven years ago", for instance, neither would Jesus' listeners.

Second, the numeral system used by Jesus' listeners was probably either Roman numerals or Greek ones. In the latter, 70 would be expressed by a single character (omikron). Without what we now call Arabic numerals, multiplication was harder.

And finally, most of them wouldn't care either way. Think of the disparities between metric and Imperial systems - and consider that the modern US system is standardized. People before the industrial era measured things chiefly in dozens, scores or grosses, and in more practical units like bundles or boxes. If someone asked for four dozen of eggs, no one would calculate 4 x 12, they'd just give them four dozens of eggs.