Answer:
The error student make is he multiply the terms instead of addition.
Explanation:
Given : A student found the product of 8 × 10 to the 6 power + 5 × 10 to the 9th power to be 4 × 10 to 15 power.
To find : What is the error ?
Solution : 
8 × 10 to the 6 power - 

5 × 10 to the 9th power - 

4 × 10 to 15 power - 

According to student, 

We solve the expression,




The error student make is he multiply the terms instead of addition.