Answer:
elastic partial width is 2.49 eV
Step-by-step explanation:
given data 
ER E = 250 eV 
spin J = 0
cross-section magnitude σ = 1300 barns
peak P = 20ev
to find out
elastic partial width W
solution
we know here that 
σ = λ²× W / ( E × π × P ) ...................1
put here all value
σ = (0.286)² × W × 
 / ( 250 × π × 20 )
/ ( 250 × π × 20 ) 
1300 × 
 = (0.286)² × W ×
 = (0.286)² × W × 
 / ( 250 × π × 20 )
/ ( 250 × π × 20 ) 
solve it and we get W
W = 249.56 × 
 
 
so elastic partial width is 2.49 eV