can someone help explain the proof of this problem?

GIVEN: INSIDE VARIABLE COST/UNIT INSIDE FIXED COSTS 28 265,000 OUTSIDE VARIABLE COST/UNIT OUTSIDE FIXED COSTS 37 224,500 REQUIRED: ANSWER PROOF BREAK-EVEN UNITS? 4,500 INSIDE OUTSIDE $ 391,000 $ 391,000 UNITS WHEN OUTSOURCING SAVES $18000? 2,500 INSIDE OUTSIDE $ 335,000 $ 317,000