If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Algebra (all content)

Course: Algebra (all content)>Unit 1

Lesson 4: Evaluating expressions word problems

Evaluating expressions with variables word problems

Learn to evaluate expressions in word problems to gain new information.
Khan Academy video wrapper
Evaluating an expression with one variableSee video transcript

Reflection question

What was Sal finding when he plugged t, equals, 10 into the expression 5, t, plus, 3?

Let's try a practice problem!

The expression 2, m, plus, 10, c gives the amount of money, in dollars, a dessert store makes from selling m muffins and c cakes.
How much money does the dessert store make from selling three muffins and four cakes?
dollars

Challenge problem

Part A
A flower store uses the expression 2, plus, 5, r to determine the cost, in dollars, of r roses.
Complete the table to find the cost of different numbers of roses.
Number of roses left parenthesis, r, right parenthesisCost left parenthesis, 2, plus, 5, r, right parenthesis
317
6
9

Part B
Cam has 32 dollars. How many roses can he afford to buy?
Assume that he wants to buy as many roses as he can.

Extra challenge

Explain to a family member, friend, or classmate why the cost of six roses is not double the cost of three roses.

Want to join the conversation?

• This extra challenge has frozen the two brain cells I had to rub together, and fried them at the same time, which I did not know was possible. There's simply not enough information to make an informed, rational decision. I mean, who's writing these challenges? 10 out of 10 for style... I think it has something to do with overhead costs and distance to travel, but I just can't sort it out with any clarity.
• The expression 2 + 5r for the total cost of r roses means $5 per rose plus a constant$2 fee for the entire purchase. If the number of roses is doubled (for example, 6 roses instead of 3 roses), only the cost of the roses without the fee is doubled, but the $2 fee is not doubled to$4. So doubling the number of roses comes $2 short of doubling the total cost. Algebraically, the total cost of 2r roses is 2 + 5(2r) = 2 + 10r dollars. However, twice the total cost of r roses would be 2(2 + 5r) = 2(2) + 2(5r) = 4 + 10r dollars. So once again, we see that doubling the number of roses comes$2 short of doubling the total cost.

Have a blessed, wonderful day!
• how do you find the value of the muffins and cakes? it doesn't give enough info?
• It gives all the info you need.
We have the expression:
2m+10c
Where m are muffins and c are cakes.

the question is : How much money does the dessert store make from selling three muffins and four cakes?

So m = 3 and c = 4
Now just plug muffins and cakes into the expression 2m+10c
2(3) + 10(4) or 2*3+10*4, both are the same thing

Now do the multiplication first and it becomes:
6 + 40
= 46

$46 dollars for 3 muffins and 4 cakes (16 votes) • the extra challenge is making me loose my brain cells (12 votes) • If the variable was multiplied by 5 only then it would double but then if you add 2 it won't double. (3 votes) • can you explain 100+20(51/4) (5 votes) • 100+20(5 1/4) Follow order of operations rules - PEMDAS: You must multiply 1st. -- Change 5 1/4 into an improper fraction = 21/4 -- Multiply: 20 (21/4) = 20/1 * 21/4 = 5 (21) = 105 Your expression is now: 100+105 Add the 2 numbers and you'll have your answer. (6 votes) • Cam has 32 dollars. How many roses can he afford to buy? Assume that he wants to buy as many roses as he can. The answer is he can buy 6 roses for$32 but I do not understand why that is the answer.
• If he buys 6 roses, the cost (from the formula in the problem) is 2 + 5r = 2 + 5(6) = 32.
• What does it mean mathematically if you "plug something in"?
• It's called "substitution". You replace one item with another of equal value.