People of Reddit, what was the toughest math question you had ever solved ?

I had a group project in high school that involved calculating the cost of re-turfing the school’s football field. It was slightly elevated in the center for water run-off, so we had to measure that, then do the calculus to find the volume under the curve at the depth we needed, and then call around to find the price for the soil. It was actually a pretty fun project that we had about a week to do.