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# Evaluating an expression with one variable

Learn how to evaluate an expression with variables using a technique called substitution (or "plugging in"). Created by Sal Khan and Monterey Institute for Technology and Education.

Video transcript

A local hospital is holding
a raffle as a fundraiser. The individual cost of
participating in the raffle is given by the following
expression-- 5t plus 3, or 5 times t plus 3,
where t represents the number of tickets
someone purchases. Evaluate the expression when t
is equal to 1, t is equal to 8, and t is equal to 10. So let's first
take the situation where t is equal to 1. Then this expression
right over here becomes-- and I'll use that
same color-- becomes 5 times 1. 5 times 1 plus 3. 5 times 1 plus 3, and we know
from order of operations, you do the multiplication
before you do the addition. So this will be 5
times 1 is 5 plus 3, and then this is
clearly equal to 8. Now let's do it when
t is equal to 8. So when t is equal to 8,
this expression becomes-- and I'll do the same colors
again-- 5 times 8 plus 3. Same color of green. And once again, 5 times 8 is
40, and then we have the plus 3, there so this is equal to 43. And so we have the
last situation, with t is equal to 10. I'll do that in blue. So we have 5 times 10. So 5t is 5 times 10. Instead of a t, put a 10 there. 5 times 10 plus 3. That's a slightly
different shade of green, but I think you get the idea. 5 times 10 is 50. We do 50, and then we're going
to have to add 3 to that, and that is equal to 53. And we're done.