Well the first thing you need to do is set up a proportion.
8 girls 56 girls
-------- = -----------
7 boys x
Now what we have to do is find out what we multiplied by 8 to get 56.
Simply divide.
56 ÷ 8 = 7
So, we figured out that the magic number is 7!
So we multiply the boy number by 7
7 x 7 = 49
So this means that there are 49 boys on the field trip.