A plant of the genotype DEF over def is test crossed with def over def…

Step 1. (a) $6 + 6 = 12$ offspring. The gene E switches in them, so E is in the middle, and the order is D, E, F.

Step 2. (b) Between D and E: the single crossovers dEF and Def ($54 + 54 = 108$) and the double crossovers ($12$). Total $120$ of $1000$, which is $12$ map units.

Step 3. (c) Between E and F: the single crossovers DEf and deF ($119 + 119 = 238$) and the double crossovers ($12$). Total $250$ of $1000$, which is $25$ map units.

Step 4. (d) Expected double crossovers: $0.12 \times 0.25 \times 1000 = 30$. Observed: $12$. The coefficient of coincidence is $12 \div 30 = 0.4$.