According to the trend in device scaling historically observed by Moore's Law, the number of transistors on a chip in 2025 should be how many times the number in 2015?

32 times.

Explanation:

Moore's law, established by Gordon Moore, one of the founders of Intel, in 1965, states basically that the number of transistors in a dense chip (like a microprocessor) approximately doubles each 2 years.

If we take a base = 100 for 2015, and double this quantity each two years, we will have that by 2025, this number will be 3,200, being effectively 32 times the number for 2015.