[QUOTE=shorter formula is this:

Gear inches times 0.00297 times cadence=speed.

just remember the .00297 and yer good.[/QUOTE]

The math doesn't work out:
Gear inches x 0.00297 x cadence= speed
(20 inch wheel *42T/20T as stated by [MENTION=313360]welshTerrier2[/MENTION]) x 0.00297 x cadence= 15 mph (as stated by OP)
42 gear inch x 0.00297 x cadence = 15 mph
cadence= 15/42/0.00297
cadence= 120
cadence is quite high