Martha bought 3 m. of yellow ribbon and blue ribbon. How many meters of ribbon did she buy?
I was first tempted to say she bought 3m of ribbon, because that's what you just said!
But, you probably meant 3m of yellow and 3m of blue.
And you're saying you cannot add 3+3 ???
To find out how many meters of ribbon Martha bought, you need to know the lengths of both the yellow ribbon and the blue ribbon. The question mentions that Martha bought 3 meters of ribbon in total, but it does not specify whether that is the combined length of both ribbons or the length of each ribbon individually.
If the 3 meters mentioned is the combined length of both ribbons, then you can't determine the exact lengths of the yellow and blue ribbons without additional information. They could be any lengths, as long as their sum is 3 meters. For example, Martha could have bought 2 meters of yellow ribbon and 1 meter of blue ribbon, or she could have bought 1.5 meters of each ribbon, or any other combination.
On the other hand, if the 3 meters mentioned refers to the length of each ribbon individually, then Martha bought 3 meters of yellow ribbon and 3 meters of blue ribbon, for a total of 6 meters of ribbon.
Therefore, without more information, it is not possible to determine the exact lengths of the yellow and blue ribbons that Martha bought.