**Answer:**

(1)The percentage error is 6.897 % .

(2)The total cost of the wallet is $34.35 .

(3)The salesman’s commission as a percent of his total monthly sales is 4% .

(4)The interest is $5400 .

**Step-by-step explanation:**

**First part **

Formula

Change in value = |Approx value – Exact value|

As given

Mark estimates that the distance from his house to his school is 15.5 mi. The actual distance is 14.5 mi.

Approx value = 15.5 mi

Exact value = 14.5 mi

Change in value = |15.5 – 14.5|

= 1

Put all the values in the formula

Percentage error = 6.897 % (Approx)

Thus the percentage error is 6.897 % .

**Second part **

The price of a wallet is $31.95 and the sales tax is 7.5%.

7.5% is written in the decimal form

= 0.075

Sales tax price = 0.075 × Price of the wallet

= 0.075 × $31.95

= $2.4 (Approx)

Total cost of the wallets = Cost of the wallet + Sales tax price

= $31.95 + $2.4

= $34.35

Therefore the total cost of the wallet is $34.35.

**Third part **

**Formula **

A sneaker store salesman had $4,125 in total monthly sales last month. He made $165 in commission from those sales.

Part value = $165

Total value = $4125

Put all the values in the formula

Percentage = 4%

Therefore the salesman’s commission as a percent of his total monthly sales is 4% .

**Fourth part **

Formula

Principle = $12000

Rate = 15%

Time = 3 years

Put all the values in the formula

Simple interest = $5400

Therefore the interest is $5400 .