Each letter is given a weight and it's multiplied by the length of the word it's part of to get its effective weight. Sum it up for all letters and you get the

Each letter is given a weight and it's multiplied by the length of the word it's part of to get its effective weight. Sum it up for all letters and you get the final score. Here, it seems like two 10 letter words can be formed using the given 20. Hint - blackboard is one such word.