We're given an assumption and data to arrive at the conclusion. First we need to calculate the heat released from cooling water,
of heat are released by cooling that water.
The equation for combusting butane is a combustion reaction,
This is where many will make a stupid mistake, there are 2 moles of butane in that reaction, so our calculations must take that into account when calculating the grams needed. I guarantee you there would be an answer on the test for this mistake.
Moving on, we need,
of butane to release the quantity of heat desired.