Mathematics · pattern A, Numeric fields
Enter two decimals for all four operations, with rounding to a chosen number of places and the decimal-place count that verifies a multiplication.
align the points for addition; add the place counts for multiplication
The number of decimal places in a product is the sum of the places in the two factors, which is the fastest check available on a multiplication.
16.15000000
Every rounding step introduces a small error, and rounding at each intermediate stage lets those errors accumulate past the tolerance of the final answer. Carry full precision through the working and round once, at the end, to the precision the question asks for.
If the number is not the part you are stuck on, that is what the service is for — a specialist who explains the working, not just the answer.
Arithmetic runs in double-precision floating point, so results beyond about fifteen significant figures are not exact. Where a question wants an exact fraction or surd, keep the exact form rather than a decimal.