A plant with round yellow seeds is crossed with a plant with round green seeds…

Step 1. Shape. Round is $3 + 3 = 6$ parts and wrinkled is $1 + 1 = 2$ parts, a ratio of $3 : 1$. So both parents are Rr.

Step 2. Colour. Yellow is $3 + 1 = 4$ parts and green is $3 + 1 = 4$ parts, a ratio of $1 : 1$. The green parent is yy, so the yellow parent is Yy.

Step 3. (a) The round yellow parent is RrYy and the round green parent is Rryy.

Step 4. (b) RrYy makes RY, Ry, rY and ry. Rryy makes Ry and ry.

Step 5. (c) Wrinkled is $\tfrac{2}{8} = \tfrac{1}{4}$ of the offspring.