The number of ways 4 boys and 3 girls are to be arranged in a row so that all 3 girls are not together, is equal to:
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Note the wording: "all 3 girls are not together" means you only subtract the case where the block of 3 is intact. It still allows for 2 girls to be together. If the question said "no two girls are together," you would use the Gap Method instead.