Let's roll up our sleeves and solve this. We have 161 and we're going to multiply it by 4, essentially adding 161 to itself 4 times.
Step 1: 0 + 161 = 161
Step 2: 161 + 161 = 322
Step 3: 322 + 161 = 483
Step 4: 483 + 161 = 644

So, 161*4 = 644