A=cost of one mango, B=cost of one orange.
3A+8B=7A+5B-12, so 4A-3B=12. From this we can relate the cost of the fruits: B=4A/3-4.
The question is puzzling because it says 7A+5B are bought "at a cost of ksh 48 each". The context for "each" needs clarification. Assume it means 7A+5B=48 or 3A+8B=36.
Then we have: 3A+8(4A/3-4)=36; 41A/3-32=36; 41A/3=68 so A=204/41=ksh 4.98 approx, so B=ksh 2.63 approx.
The combined cost of a mango and an orange=ksh 7.61.