First, we find the value of the discount.
20% = 20/100
20/100*$150= $30 (20% off)
Then we subtract the discount from the original price: $150 - $30 = $120
Finally we find the value of the tax and we add it to the last value ($120)
15% = 15/100
15/100*$120=$18 (15% Tax)
$120 + $18 = $138
The final value would be equal to $138.