Good explanation that demonstrates good knowledge of the ratios. However, if you can't make the connection, you can go about it GMAT style:
Here is what you know:
1) ROA= NI/Assets= 0.20
2) Margin= NI/Sales= 0.05
3) TAT= Sales/Assets= ?
Here's what you can do:
1) plug any number for assets (100 is a good one)
2) If Assets are 100, than NI must be 20 (to have ROA of 20%)
3) Given that NI is 20, you know that Sales must be 400 (to have your margin at 5%)
4) Just plug your numbers in TAT: 400/100= 4
Hope this helps