They quickly realized it would be much faster to count 10 tens, or 100, and show this using the flats. As they completed this proof, they laid out 6 flats and 4 longs, or 600 + 40. This hands on strategy is a great way to use a concrete tool for computation of a tricky number!
The above student used a very similar strategy, but showed her thinking using the pictorial form. She counted the tens under each symbol for base-10 blocks.
pictured is his thinking that included using a number bond to look at 64 tens as 60 and 4 tens.