Question

The answer is 5:6:20 but can someone explain how plz?

Answer 1

The ratio of red to green is 5:6 which means that for every 5 red cars, there are 6 green cars

The ratio of green to blue is 3:10 telling us that for every 3 green cars, there are 10 blue cars. 

The ratio 3:10 is equivalent to 6:20 after we multiply both parts by 2. This now says that for every 6 green cars, there are 20 blue cars.

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Let's say we had 5 red cars, 6 green cars and 20 blue cars
Based on that info, we know that the ratio of red to green is 5:6
And the ratio of green to blue is 6:20 which reduces to 3:10

We don't reduce 6:20 to 3:10 however, since that would change the green count from 6 to 3. We want to keep the green count at 6.

So because there are 5 red cars, 6 green cars, and 20 blue cars in this example, and this example points to the proper ratios mentioned earlier, this means that the final answer is 5:6:20. This ratio cannot be reduced or simplified as there are no common factors (other than 1) for 5, 6, and 20.