To solve this question, simply know that Betty would need 4 and 1/3 balls of yarn for 7 scarves, so if she needs to make twice as much, she would also need twice as much balls of yarn, thus she would need 2 X 4 1/3 or 2 X 13/3 = 26/3 = 8 2/3. Likewise do the same for 21 scarves, so it would be 3 times as much for balls of yarn, thus it would be 3 X 4 1/3 or 3 X 13/3 = 39/3 = 13.