How do u do this? ! plz explain to me (#5)

Ateacher brought a pack of 17 batteries . each calculator uses 3 batteries . how many calculator can the teacher fill with batteries

Aright triangle has an area of 21 m2. the dimensions of the triangle are increased by a scale factor of 4. what is the area of the new triangle

At noon a tank contained 10cm water. after several hours it contained 7cm of water. what is the percent decrease of water in the tank?