USING NEGATIVE INTEGERS IN REAL LIFE

Example 1 : 

Hannah made four withdrawals of $20 from her checking account. She also wrote a check for $215. By how much did the amount in her checking account change ?

Solution : 

We have to find the total change in Hannah’s account. Since withdrawals and writing a check represent a decrease in her account, use negative numbers to represent these amounts.

Step 1 :

Write a product to represent the four withdrawals.

-20 + (-20) + (-20) + (-20)  =  4(-20)

Add -215 to represent the check that Hannah wrote.

4(-20) + (-215)

Step 2 :

Evaluate the expression to find by how much the amount in the account changed.

4(-20) - 215  =  -80 - 215 (Multiply first)

=  -295 (Then, subtract)

(The value -295 represents a decrease of 295 dollars)

So, the amount in the account decreased by $295.

Example 2 : 

Three brothers each have their own savings. They borrow $72 from their parents for concert tickets. Each brother must pay back an equal share of this amount. Also, the youngest brother owes his parents $15. By how much will the youngest brother’s savings change after he pays his parents ?

Solution : 

We have to find the total change in Hannah’s account. Since withdrawals and writing a check represent a decrease in her account, use negative numbers to represent these amounts.

Step 1 :

Determine the signs of the values and the operations you will use.

Write an expression.

Since the money is being paid back, it will decrease the amount in each brother’s savings. Use -72 and -15.

Since an equal share of the $72 will be paid back, use division to determine 3 equal parts of -72. Then add -15 to one of these equal parts.

Change to youngest brother’s savings = (-72) ÷ 3 + (-15)

Step 2 :

Evaluate the expression.

(-72) ÷ 3 + (-15) = -24 + (-15)  (Divide first)

=  -39 (Then, add)

(The value -39 represents a decrease of 39 dollars)

So, youngest brother’s savings will decrease by $39.

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