Let's dissect this problem: "A textbook costs $58" Okay, original amount: $58.
"and sells for $70" Okay, new price: $70. "Find rate of markup" Alright!
So to set up the problem, you need to find $58 times some number is $70.
That looks like this:
$58 * SN = $70
Now divide $70 by $58
SN = 1.21
Now make it a percentage:
121%
Now subtract 100% because it is markup
21%
The markup on the textbook is 21%.
Hope that helps!