Answer:
Each friend ate 4 tacos.
Step-by-step explanation:
"4 of his friends ate s tacos" is unclear. I assume it should be
"4 of his friends ate s tacos each."
total tacos = 2 + 4s = 18
4s = 16
s = 4
His four friends ate a total of 16 tacos, each friend eating 4 tacos.
Ricky's reason, i.e., that 6s = 18, is incorrect.
6s = 4s+2s, where 4s is what his friends ate in total.
That implies that Ricky ate 2s tacos, instead of 2 tacos. If 2s = 2 then s = 1, but then 6s ≠ 18.
