Answer:
Carlos subtracted 2 tens, not 3 tens.
Explanation:
In step 1, there are 90 blocks divided into 9 tens. However in step 2, Carlos mistakenly subtracted only 2 tens which is equivalent to 20 blocks. It should be 3 tens which is equivalent to 30 blocks. Although, he is right in subtracting 4 ones which is equivalent to 4 blocks.
1 ten = 10 blocks
2 tens = 20 blocks
3 tens = 30 blocks
and so on...
So, Carlos should subtracted 3 tens and 4 ones.
