Digital fuel sensor pulse divider

Hi, I'm new to arduino and new to this forum.

I received my 1st arduino micro today and I would like to use it to divide pulses from a fuel flow sensor.

Here is the situation... I have a fuel meter and 2 different fuel flow sensor (they are open collector) The 1st one is on the fuel inlet the other on the return to tank line.

1st one has 29000 pulses per gallon and the return one 69000 pulses per gallon.

I need to receive the pulse from the return sensor and repeat it at 42%... which means only 42% of the digital input from the sensor are sent to the output (The meter).

Could anyone guide me in the right direction... I have no previous knowledge of Arduino programing.

Cheers!
Marc