The problem asks for the total number of songs so let "n" be that number.
"Girls alone perform 50% minus eight of the songs". That will be 0.50n- 8.
"Boys alone perform 20% plus seven of the songs". That will be 0.20n+ 7.
"They also perform 13 songs all together."
So they performed a total of (0.5n-
+ (0.2n+ 7)+ 13= 0.7n+ 12 songs. But we have already said that the number of song performed was n: 0.7n+ 12= n. Solve that equation for n.