the answer is x=4
this is because you add 4 to both sides then divide both sides by 5 to get 4
During week 2, he could type 15 words per minute, at his fourth week, he could type 30. So 30-15=15. He could type 15 words faster on week 4 than he could at week 2.
10 x 40 = 400 so the answer is 400 pieces of chalk.