The answer is uhmm let me see 2 4 6 8 10 12 14 16 18 20. Oh the answer is 4
1x+2x = 18
3x=18
x=6
there are 6 pieces of yellow chalk
If 4x = 8x - 1 then 4x must be equivalent to 1. So that means x is .25 or 1/4.
Hope this helps!
(I assumed your trying to solve for x, please word the question better next time)