The first-law sign convention is stated

A fictional closed gas system receives 500 joules of heat during a classroom process and does 180 joules of work on its surroundings. The supplied convention is change in internal energy equals heat added minus work done by the system. A student adds both positive numbers without examining the convention, while another writes each transfer direction explicitly before substituting. The problem supplies no chemical reaction, phase-change complication or real plant operating instruction. This written exercise changes no pressure or temperature. It asks for the energy balance, the meaning of the resulting sign and an explanation of why using a different work convention requires changing the equation consistently rather than changing just one symbol.