Ion drive for spacecraft

The ion drive of the Deep Space 1 spacecraft, pictured on page 76 and discussed in example 2, produces a thrust of 90 mN (millinewtons). It carries about 80 kg of reaction mass, which it ejects at a speed of 30,000 m/s. For how long can the engine continue supplying this amount of thrust before running out of reaction mass to shove out the back?

Solving the equation F = Δp/Δt for the unknown Δt, and treating force and momentum as scalars since the problem is one-dimensional, we find