Correct option is : (2) 70
\(\text { average speed }=\frac{\text { distance }}{\text { time }}\)
\(=\frac{\frac{1}{2}(\mathrm{t})(80)+80 \times 3 \mathrm{t}}{4 \mathrm{t}}\)
\(=\frac{40 \mathrm{t}+240 \mathrm{t}}{4 \mathrm{t}} \)
\(=\frac{280}{4}=70 \mathrm{~km} / \mathrm{hr}\)