1)

Let x be the number of 24 cent stamps
Let y be the number of 39 cent stamps

Then we need to find the integer solution to the following equation 24x+39y=300

rearranging, we get y=(300-24x)/39

which is equivalent to y=(100-8x)/13

You then have to find the different integer values of x that yields an integer value for y, in this case, x=6 which yields y=4

So you need six 24 cent stamps and four 39 cent stamps.