Answer:
68 pounds.
Step-by-step explanation:
Find the total weight when there are 10 boys and when there are 9 boys then calculate the difference.
I'll let x = total weight.
[tex]\frac{x}{10}=77\\(10)\frac{x}{10}=77(10)\\x=770\\\\ \frac{x}{9}=78\\(9)\frac{x}{9}=78(9)\\x=702\\\\ 770-702=68[/tex]
Therefore, the youngest boy weighs 68 pounds.