Answer:
Nina practiced the viola for 11.25 hours last week.
Explanation:
Juan practiced the violin for 9 hours last week. For each 3 hours that he practices the violin, he practices 2.5 hours of cello. So
3h violin - 2.5h cello
9h violin - xh cello
3x = 9 * 2.5
x = 7.5
He practiced 7.5 hours of cello, the same as Nina.
For every 3 hours Nina spends practicing the viola she practices the cello for 2 hours. So:
3h viola - 2h cello
xh viola - 7.5h cello
2x =3 * 7.5
x = 7.5 * 1.5
x = 11.25
Nina practiced the viola for 11.25 hours last week.