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A rocket with initial mass 70000kg burns fuel at a rate of 250kg/s; it has an exhaust velocity of 2500m/s. If the rocket is at rest, how long after the engines fire will the rocket lift off?

I've been trying to solve it using some variation of the rocket equation:

v=v(exhaust) * ln(M(0)/M(t)) - gt

but to no success.

Any hints or help would be greatly appreciated