Secret Sum Detective: Deduce Every Hidden Cell From Row and Column Totals

The Hard Part Isn't the Subtraction — It's Knowing Which Hidden Cell to Solve First

Generate fresh grid values, revealed positions, and row/column sum clues. Keep givens fixed and enter values satisfying every sum; alternative legal fills are accepted and New set changes clues.

From One Missing Addend to a Chain of Missing Addends: Grade 5-6

A single line — target minus the revealed parts — is exactly the missing-addend equation x + p = q from 6.EE.B.7, and Secret Sum Detective strings those equations together so each solved unknown feeds the next. Students rely on multi-digit addition and subtraction fluency from 5.NBT.B.5 to keep every subtotal exact, because one slip early in the chain topples every deduction that depends on it. Naming each masked cell as its own unknown draws on the variable-and-expression reasoning of 6.EE.B.6, and the habit of always attacking the most-constrained line first is the same order-of-attack instinct that later powers systems of equations.

Three Skills Practiced in Secret Sum Detective

• subtracting known parts from a target total
• combining horizontal and vertical clues
• eliminating candidates through cross-checks

Teacher and Parent Note

Have the student write the known subtotal and remaining difference beside each line. After finding a hidden value, require a second check in the perpendicular direction.