if jacob's age is 12,

12 = 3B

4 = B, so the brother is 4

how old will jacob be when he is twice as old as his brother?

Let that be x years from now. So in x years from now,

Jacob will be 12+x and his brother will be 4+x

12+x = 2(4+x)

12+x = 8+2x

4 = x

So that will be 4 years from now

So Jacob will be 12+4 or 16 and his brother will be 4+4 or 8.

Answer: Jacob will be 16.