Dear Omeid, this is a homework problem, right? I think we shouldn't be answering the question "explicitly".
But let me mention a hint: if you assume that the data are enough to calculate a unique answer, do you think that the information about the length of the bed may be relevant? You want to get a result in kilograms - and there are two pieces of information in kilograms and one in centimeters. To get a result in kilograms, may the figure involving centimeters matter if you require that the result has the right units?
This argument is called "dimensional analysis".
Now, imagine that you put both scales under the bed simultaneously: one of them is under the foot and one of them is under the head - at the same moment. How much do you think that those scales will show in this case? How can you determine the total ;-) mass of the bed using this measurement?