Jonathan has a collection of 400 marbles. blue marbles make up 17%, percent of his collection. how many blue marbles does jonathan have?

17% of 400
percent means parts ou of 100
17%=17/100
'of' means mutiply

17/100 tiems 400=4*17=68

68 blue marbles

To find any percentage of something, just multiply the whole value by the percentage of whatever you're trying to find the value of. 
17% converted to a decimal is 0.17. 400*0.17 = 68.
Jonathan has 68 blue marbles in his collection.
Hope that helped you!