The capstone keeps strain, elastic stress, and pressure force typed. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Reuse strain in the elastic stress check
The audit reuses strain 1/2 and modulus 12 Pa to source stress 6 Pa.
12⋅21=6
Pressure force is a separate checked source
The pressure-force source is 18 N from 6 Pa over 3 square metres.