Blocks A and B are identical metal blocks. Initially block A is neutral, and block B has a net charge of 7.0 nC. Using insulating handles, the blocks are moved so they touch each other. After touching for a few seconds, the blocks are separated (again using insulating handles). What is the final charge of block A?

3.5 nC

Explanation:

When both metal blocks get in touch each other, as they are assumed perfect conductors, excess charge is evenly spread out on the surfaces on both blocks.

Due to the total charge must be conserved, this means that if one block had a charge Q, and the other block was neutral, after separated, each of them must have exactly the half of the total excess charge.

So, if the total charge was 7 nC, each block must keep 3.5 nC, i. e., the final charge of block A is 3.5 nC.