The condition says NorthAmerica + Europe + Asia = 21, NorthAmerica - (Europe + Asia) = 5. Add the two equations. You will get 2*NorthAmerica = 21+5 = 26 ====> NorthAmerica== 13. Then (Europe + Asia) = 21 - 13 = 8. Next, you have these two equations Europe + Asia = 8, Europe - Asia = 4. Add the two equations. You will get 2*Europe = 8+4 = 12 ===> Europe = = 6. Then Asia = 8-6 = 2. Answer. NorthAmerica = 13, Europe = 6 and Asia = 2.