5. The amount of rice in warehouses A and B is the same. Dad, Mom, and Yangyang can each transport a warehouse of rice alone in 10 days, 12 days, and 15 days, respectively. Dad and Mom start transporting the rice from warehouses A and B simultaneously, and Yangyang first helps Mom, then helps Dad, and they finish transporting the rice from both warehouses at the same time. How many days did Yangyang help Mom? 
A. 3
B. 4
C. 5
D. 6
Analysis: Engineering problem.
Three people moved a total of $(1+1) \div\left(\frac{1}{10}+\frac{1}{12}+\frac{1}{15}\right)=8$ days, Yangyang helped his mother transport 1
Father, mother, and Yangyang worked together to move the rice from two warehouses, assuming the amount of rice in one warehouse is "1", $\left(1-\frac{1}{12} \times 8\right) \div \frac{1}{15}=5$ days, the answer is $\mathrm{C}$.