Mary is caring for 25 animals while her neighbors are on vacation. The number of cows is 3 greater than the number of pigs. How many pigs is she taking care of?

To solve, you make a systems of equation:

x + (x+3) = 25, the x representing the number of pigs. Since there is no multiplication and following the order of operations, you add the x's to get 2x+3=25. Then, you subtract 3 from both sides to get 2x=22, then divide by 2 on both sides to isolate the variable. Therefore, x=11 and Mary is taking care of 11 pigs.