please help me with these business math equations.

4. Ahmed wishes to have a return of 5.5% on his two investments annually. If he has SAR 10,000 invested at 5%, how much additional money should be invested at 8% ? 5. Mr. Al-Qahtani has twice as much money invested at 9% than at 5%. If the annual income of the two investments is SAR1,610, how much is invested at each rate? 3. A buyer of an appliance store bought a supply of electric fans at SAR100 per electric fan. One-half in the supply of these electric fans were sold at SAR115 each and the other half were sold at SAR125. If the gross profit was SAR1,600, how many electric fans were in the supply