Mary had a number of cookies. after eating one, she gave half the remainder to her sister. after eating another cookie, she gave half of what was left to her brother. mary now had only five cookies left. how many cookies did she start with

For this question, you have to work backwards, so if she ate a cookie, working backwards, you would have to add, not subtract or divide.

Mary had 5 cookies after she gave half away. So 5 is half of something. 5 x 2 = 10. She had 10 cookies before she gave half to her brother.

Before she had 10, she ate one. So you add one to the ten.(Remember, working backwards). 10+1=11.

She had 11 cookies after she gave half. So 11 x 2 = 22.
She had 22 cookies before she gave half away.
She ate one before she had 22. 22+1=23.
We are finished. Mary started with 23 cookies.

Hope you understand! :)