Carla will do 525 jumping jacks in 7 days.
First, you multiply 25 by 3. You get 75.
Then you have to multiply 75 by 7. You get 525.
You multiply 25 by 3 to get 75 because that's how many she does in 1 day. And you multiply 75 by 7 to get 525 because she does 75 jumping jacks per day, for 7 days.