This study reports on the use of formative, diagnostic online assessments for the topic percentages. Two new item formats (drag-drop and slider) are described. About one-third of the school students (Years 7 to 9) could, using a slider, estimate "80% more than" a given length, in contrast with over two-thirds who could estimate "90% of" a given length. While four-fifths of the school students could, using drag-drop cards, choose the 2-step calculation of a reduced price after a 35% discount, only one-third could choose the corresponding 1- step calculation.