To find the total cost of the equipment, you can calculate the actual cost of the materials and then apply the discount.
1. Cost of ping pong ball + cost of shuttlecocks
(0.35 x 24) + (2.75 x 4)
$19.40
2. 100% - 30% discount = 70% of it you are actually paying for. Change the percent to a decimal to calculate
0.70 x $19.40 = $13.58
The cost will be $13.58.