I use a 5-step process when working with word problems: (1) read and understand the problem; (2) translate the words into mathematical equations and expressions; (3) check and make sure that I understand what I am solving for; (4) solve the problem; and (5) check to see if the answer makes sense If not, redo all the steps.
Step 1: We are being asked to find out how much money Jenny withdrew from the bank.
Step 2: To translate the words into mathematical equations and expressions, we need to know what we do not know. We do not know how much money Jenny withdrew from the bank. Let make "x" the amount of money that Jenny withdrew from the bank. Here are things that we know:
x (amount of money that Jenny withdrew from the bank)
0.10x (amount Jenny gave to her sister)
0.90x (amount Jenny kept after giving 10% to her sister)
(0.90x)*(1/3) (amount Jenny spend on a book)
72=(0.90x)*(2/3) (amount Jenny gave to an animal shelter)
Step 3: We are solving for "x", which is the amount of money that Jenny withdrew from the bank
Step 4: Solve for "x". Notice that we have one equation and one unknown [72=(0.90x)*(2/3)]. Therefore, we are able to solve this problem. When you solve for x, you should receive:
x = 72*(3/2)/(.90)
x = 120 dollars <===== ANSWER
Step 5: To see if this answer makes sense, let's attempt to see if we can get the amount giving to the animal shelter, $72.
We know that the Jenny withdrew $120 and gave 10% (or $12) to her sister. With $108 dollar Jenny kept, she spent 1/3 (or $36) on a book, which left her with $72 to give an animal shelter.
Since we obtain $72, our answer makes sense.