Build a shared-expense tracker. Three people split a bill of ten pounds evenly - write the classes, and tell me where the missing penny goes.

Short answer

In a Splitwise-style machine coding round, the core invariant is that every expense split must add back to the exact expense total. Store money as integer minor units or an exact decimal type, never double, and allocate any leftover penny by a deterministic rule. The strongest design keeps balances derived from expenses and validates the sum-to-total invariant in one place.

Keep machine coding explicit in the answer because that is the concept the interviewer is actually trying to test. A good machine coding explanation names the trade-off, the failure mode, and the evidence you would use before choosing.