Answer
You should buy Brand K.
Work Step by Step
We compare the price (per tissue) of Brands K and S. We use a calculator for the division.
Brand K:
$\$5$ for $3$ boxes: $\$5\div3=\$1.67/box$
$\$0.45$ coupon for 1 box: $\$1.67-\$0.45=\$1.22/box$
$175$ tissues/box: $\$1.22/175\approx \$0.007/tissue$
Brand S:
$\$1.29$ for $1$ box: $\$1.29/box$
$\$0.20$ coupon for 1 box: $\$1.29-\$0.20=\$1.09/box$
$125$ tissues/box: $\$1.09/125\approx \$0.009/tissue$
Therefore, Brand K is $\approx \$0.002/tissue$ cheaper than Brand S. You should buy Brand K.
