Using the numbers [10, 20, 30, 40], create an equation that equals 168. You can use basic arithmetic operations (+, -, *, /) and each number can only be used once. Show your work in <think> </think> tags. And return the final equation and answer in <answer> </answer> tags, for example <answer> (10 * 20) + (30 * 40) = 1680 </answer>.