Answer:
2.1 centimeters thick.
Explanation:
If he stacks them you can think of it in 2 ways:
1. Keep adding 0.3 cm until you add 7 times.
2. Multiplication, 0.3 multiplied by 7 will get you your answer!
Let's try the first way.
0.3+0.3=0.6+0.3=0.9+0.3=1.2+0.3=1.5+0.3=1.8+0.3=2.1
1 2 3 4 5 6 7
Now the second way.
0.3*7. Stacking them and then doing normal multiplication gets you 2.1 as well! And as a plus, it is much faster than adding and adding over and over.
So there you go! 2.1 centimeters. Hope this helped!