215. Shopping Cart Discount Calculator

Write a function solve that calculates the total cost of items in a shopping cart after applying discounts. Items are [price, quantity]. Apply discounts in order: 10% off if total > $100, $20 off if total >= $200, buy-2-get-1-free on quantities ≥ 3.

Example:
Input: [[25.0, 4], [15.0, 2]]
Output: 94.5

Run this challenge interactively in Python or JavaScript when the app loads.