30% of $15.40 is ( 0.3 x $15.40) = $4.62 .
From the way the question is written, we don't know if
that $4.62 is the discount or the sale price.
Is the calculator on sale for 30% ?
Or is it on sale for 30% OFF ?
If 30% is the discount, then the amount of saving is $4.62 .
If 30% is the sale price, then the amount of saving is ($15.40 - $4.62)
= $10.78 .