Marty used the subsequent mathematical statement to show he could change an expression and still get the similar answer on both sides:
10 × (6 × 5) = (10 × 6) × 5
Which mathematical property did Marty use?
Using the simple interest formula Interest = principal × rate × time, or I = prt, substitute I = $4,800, p = $12,000, and r = .08 (the interest rate as a decimal); 4,800 = (12,000)(.08)(t). Multiply 12,000 and .08 to get 960, so 4,800 = 960t. Divide both sides by 960 to get 5 = t. Therefore, the time is 5 years.