Jim can fill a pool carrying buckets of water in 30 minutes

Jim can fill a pool carrying buckets of water in 30 minutes. Sue can do the same job in 45 minutes. Tony can do the same job in 1

they each fill a FRACTION of the pool, based on their individual rates
___ the fractions sum to one -- the WHOLE pool

(x / 30) + (x / 45) + (x / 90) = 1

multiplying by 90 (LCD) ___ 3x + 2x + x = 90

6x = 90 ___ x = 15