15. On Tuesday, Shanice bought 5 new pens. On Wednesday, half of all the pens that she had were accidentally thrown away. On Thursday, there were only 16 left. How many did she have on Monday? Let $p$ represent the number of pens she had on Monday. Equation or inequality: Shanice had pens on Monday

Step 1 ：Let p represent the number of pens Shanice had on Monday. After buying 5 new pens on Tuesday, she had (p + 5) pens. On Wednesday, half of her pens were thrown away, so she had (p + 5)/2 pens left. On Thursday, she had 16 pens, so we can set up the following equation: \(\frac{p + 5}{2} = 16\)

Step 2 ：To solve for p, we can multiply both sides of the equation by 2: \(p + 5 = 32\). Then, subtract 5 from both sides: \(p = 27\).

Step 3 ：\(\boxed{27}\)