Prove that if $a_1\leq b_1\leq c_1$ and $a_2\leq b_2\leq c_2$, then $a_1+a_2\leq b_1+b_2\leq c_1+c_2$.

Lesson Parent: 
Problem: 

Prove that if $a_1\leq b_1\leq c_1$ and $a_2\leq b_2\leq c_2$, then $a_1+a_2\leq b_1+b_2\leq c_1+c_2$.

Answer: 

It is true that if $a_1\leq b_1\leq c_1$ and $a_2\leq b_2\leq c_2$, then $a_1+a_2\leq b_1+b_2\leq c_1+c_2$.