Why Battery capacity is rated in ampere-hours, which is literally current multiplied by time. Rearranging, time in hours equals amp-hours divided by the average current drawn in amperes. For example, a 10 Ah battery supplying an average 2 A lasts about 10/2 = 5 hours (less in practice, since high discharge rates and internal resistance reduce usable capacity).
Watch out Dividing current by capacity inverts the formula and gives units of 1/hours, not run time. Internal resistance plays no part in the basic capacity calculation; it affects voltage sag and efficiency under load.
Amp-hours divided by amps leaves hours. Let the units tell you which way the division goes.