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How long does the pump stay on for?
What are the current/voltage characteristics of the pump?
How accurate does the timing need to be?
What considerations are there regarding the power supply for your circuit?
How 'simple' are you thinking? Does this imply 'cheap'?
You could do something wth a 555 and a suitable transistor to drive the pump.
Or you could use a cheap MCU to get better control of the on and off times.
Lots of options but you needs to start with a proper specification.