Question

Each marble bag sold contains 4 green marbles for every 3 blue marbles if a bag has 21 blue marbles how many green marbles does it contain

Answer 1

4/3 = x/21
So you can cross multiply and divide:
3x = 84
x = 28

Or you can say 3 times what equals 21? The answer is 7. Then multiply 4 times 7 to get the answer. Whichever makes more sense to you :)