Answer and Explanation:
The computation is shown below;
a. The return on investment is 
= Margin  turnover 
= Net operating income ÷ sales ×  sales ÷ average operating assets 
For Osaka 
= $672,000 ÷ $9,600,000 × $9,600,000 ÷ $3,200,000
= 21%
For Yokohama
= $2,340,000 ÷ $2,600,000 × $2,600,000 ÷ $13,000,000
= 18%
2. The residual income is 
 = Net operating income - (average operating assets × rate of return)
For Osaka 
= $672,000 - ($3,200,000 × 16%)
= $160,000
For yokohama
= $2,340,000 - ($13,000,000 × 16%)
= $260,000