Final answer:
To find the percentage of large buttons, divide the number of large buttons by the total number of buttons and multiply by 100.
Step-by-step explanation:
To find the percentage of large buttons, we need to divide the number of large buttons by the total number of buttons (large + small) and then multiply by 100. In this case, Lacey bought 351 large buttons and 99 small buttons, so the total number of buttons is 351 + 99 = 450. Therefore, the percentage of large buttons is (351 / 450) * 100 = 78%, rounded to the nearest whole number.