A fixed strain isolates the modulus as the source factor for linear elastic stress. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Four pascals at one half strain gives two pascals
Keep strain fixed at 1/2. A toy modulus of 4 pascals gives stress 2 pascals.
4⋅21=2
Eight pascals doubles the stress
At the same strain, modulus 8 pascals gives stress 4 pascals. Only the modulus changes between these rows.
8⋅21=4
Twelve pascals gives the six-pascal audit stress
The book's base row uses modulus 12 pascals and strain one half, giving stress 6 pascals. This is a toy linear relation, not a tissue law.