At dan's school,6 out of every 10 kids know how to swim. if there are 350 kids in his school, how many know how to swim?

210 because if you multiply 10 by 35 to get 350 you have to multiply 6 by 35 then you get 210 

If the ratio of kids who know how to swim is 6:10, that means that 60% of the schools population knows how to swim.  So you can multiply 350 by 0.60 to get 210 kids.  

You can show why the calculation is true since 6/10 = x/350.  We can say the two ratios are equal since 6/10 ratio holds true for every group of 10 that leading up to a total number of 350.  (you can have 35 groups of 10 each one having a 6 to 10 ratio.  (6/10)x(35/35)=210/350