If you think about it, the ratio of 5:2, is basically saying that for every 7 people you have, 5 will be children, and 2 will be adults, or 5/7 : 2/7. So you can set it up as a proportion. If you are looking for the number of children (5 for every 7), you could as 5/7 = x/63000. If you did cross-multiplication (5*63000 = 7*x), you would get that 7x = 315000. If you them divide both sides by 7 to get the variable by itself, you would get that x is equal to 45000. This means that out of the 63000 people in the park, 45000 of them are kids.