He would have 21 before he bought the 10 stamp because 31-10=21
The equation of motion is
s(t) = (1/2)a·t² +v₀·t +s₀
where a is the acceleration, v₀ is the initial velocity, and s₀ is the initial position.
For t in seconds and s(t) in feet, your values make this equation be
s(t) = 5t² -20t +8
The answer is highlighted and I put the work for checking the answer at the bottom
Answer:

Step-by-step explanation:
Given :
QR⊥PT

To Prove : 
Solution:
Statements Reasons
QR⊥PT Given
∠QRP and ∠SRT are right angles Def of perpendicular
∠QPR≅∠STR Given
∠QRP = ∠SRT All right angles are equal
ΔPQR≈ΔTSR AA similarity
Hence 