Answer: 24
Step-by-step explanation:
Let the total number of problems be x.
After 15 minutes, [tex]x-\frac{1}{4}x=\frac{3}{4}x[/tex] problems are left.
After another 15 minutes, [tex]\frac{3}{4}x\left(1-\frac{1}{3})=\frac{3}{4}x\left(\frac{2}{3} \right)=\frac{1}{2}x[/tex] problems are left.
This is equal to 12, meaning x = 24.
