What is the 1st mistake...

Answer:
[tex]\huge\boxed{Step \ 3}[/tex]
Step-by-step explanation:
In Step # 3, We need to divide rather than to subtract. So, the first mistake is done in step 3.
Answer:
[tex]\Large \boxed{\mathrm{Step \ 4}}[/tex]
Step-by-step explanation:
[tex]20 +20 \div 4-2[/tex]
Division should be performed first, not subtraction.
[tex]20+5-2[/tex]