The answer to your question is he bought 11 pounds of cashew.
Total pounds = 16
peanuts = p = $2.75
cashew = c = $3.25
total cost = $49.50
Process
1.- Write two equations to solve this problem
p + c = 16 Equation 1
2.75p + 3.25c = 49.50 Equation 2
Solve the system by substitution
2.- Solve equation 1 for p
p = 16 – c
Substitute p in equation 2
2.75(16 – c) + 3.25c = 49.5
Expand
44 – 2.75c + 3.25c = 49.5
Solve for c
3.25c – 2.75c = 49.5 – 44
0.5c = 5.5
c = 5.5/0.5
c = 11
3.- Find p
p = 16 – 11
p = 5 pounds
4.- Conclusion
Elijah bought 11 pounds of cashew and 5 pounds of peanuts