The bridge, backwards

$15 - 8$, and count it out loud. Actually do it — say the numbers.

Now count how many times you said a number before you got to the answer. That number is the point of this lesson.

Counting back eight takes eight steps, and each one is a place to lose your thread. $15 - 5$ is not slow, because it lands on ten — and ten is a number you can stop at without thinking.

Fifteen take five is ten. That used five of the eight, so three are left: $10 - 3 = 7$. Two easy steps instead of eight risky ones.

That is :bridging, and it is the same move as the last two lessons walked backwards.

$15 - 8$ and $8 + \square = 15$ are one picture read two ways, which is module 1 and has not stopped being true.

So use whichever you know. Eight needs two to reach ten and five more to reach fifteen: seven. That is the bridge again, going up.

$85 - 8$. Drop five to eighty, then three more: seventy-seven. Counting back is hopeless here and stopping at a ten does not care.

Notice you did not think about the eighty once. So: why is eighty somewhere you can stop, when eighty-three is not?