Mrs. li bought some crackers. she gave 30% of the crackers to her neighbor and ate 50% of the remaining crackers. mrs. li had 42 crackers left. how many crackers did mrs. li buy?

She bought 120 crackers

Explanation:
Assume that the number of crackers she originally bought was x

We know that she gave her neighbor 30% of the crackers.\This means that:
amount left with her = x - 30%x
amount left with her = x - 0.3 x
amount left with her = 0.7x crackers

Now, we know that she ate 50% of that amount.
This means that:
she ate 50%(0.7x) = 0.5(0.7x) = 0.35x crackers
amount left with her = 0.7x - 0.35x = 0.35x crackers 

Finally, we know that the number of crackers left with her is 42 crackers.
This means that:
0.35x = 42
x = 
x = 120 crackers

Hope this helps :)