use divisibility rule.  3+ 7+8+R = divisible by 3.  18 + R = divisible by 3.  smallest possible number for R is 0, max possible amount for R is 9  3x6=18 - 18 =0 (can be divisible by 3)  3x7=21 -18 = 3  3x 8= 24 -18 =6  3x 9= 27 - 18 = 9  Total = 4 ways. 
If the four-digit number $3P78$ is divisible by $3$, how many possible values are there for $P$? 

[a] $$4$$ 
[b] $$3$$ 
[c] $$5$$ 
[d] $$10$$ 
[e] None of the above 