Adding & Subtracting to 100: 1st grade math in Costa Rica

Adding ones and tens to 2-digit numbers, 10 more and 10 less, and adding and subtracting within 100.

Hang out with sloths and tree frogs in steamy rainforests and misty cloud forests. Characters here greet you with “¡Pura vida!” (hello in Costa Rica (it means “pure life”)), and eggs hatch into hybrids of animals like three-toed sloth, Hoffmann's two-toed sloth, mantled howler monkey and Geoffroy's spider monkey.

What your child practices

Follows Number & Operations in Base Ten — Adding and Subtracting (1.NBT.C).

• Adding ones to a 2-digit number

  Add a few ones to a 2-digit number, like 34 + 5.

  1.NBT.C.4

• Adding tens

  Add tens to a number, like 34 + 20, by counting on in tens.

  1.NBT.C.4

• 10 more and 10 less

  Find 10 more or 10 less than a number in your head.

  1.NBT.C.5

• Subtracting tens

  Take tens away from tens, like 70 − 30.

  1.NBT.C.6

• Adding within 100

  Add two 2-digit numbers: ones with ones, tens with tens.

  1.NBT.C.4

• Subtracting within 100

  Take away from a 2-digit number, breaking a ten when you need more ones.

  2.NBT.B.5

• Story problems to 100

  Read a short story and decide: add or take away?

  1.NBT.C.4, 2.OA.A.1

Try a problem from Costa Rica

Every problem is a short story set in the world. These are real problems from the game. Kids get endless new ones with different numbers.

Common mistakes, and how to help

Every wrong answer in Mixmonde is planned: it points to a named mistake, and the tip after a miss explains it with that problem’s numbers. These are the mistakes this world watches for, grouped the way the grown-ups’ dashboard shows them.

• Place value

  The digits are right but their places are not: a 3 in the tens is worth 30, not 3. This shows up when carrying, borrowing, lining up numbers or placing a decimal point.

  Mistakes we catch: added ones to the tens, forgot the new ten, added tens as ones, added only one ten, changed the ones, took away ones, not tens, forgot to carry, wrote both sums side by side, forgot the ones, smaller from larger and broke a ten but kept it.

  Try at home: Make bundles of 10 with straws or pencils and a rubber band. Build numbers like 34 (3 bundles, 4 loose) and swap a bundle for 10 loose ones when you need to take some away.

• Counting and sequences

  Losing track while counting: off by one, skipping or double-counting, or counting the wrong things (fence posts instead of gaps).

  Mistakes we catch: counted the start number.

  Try at home: Count objects by touching or moving each one into a pile. Count on from a number other than 1 (“start at 8 and count 5 more”), and count back too.

• Order and direction

  The right idea done the wrong way round: taking the smaller digit from the bigger one, dividing backwards, swapping x and y, or flipping more and less.

  Mistakes we catch: went the wrong way.

  Try at home: Use “more” and “fewer” in everyday talk: which plate has more grapes, how many fewer? Line things up side by side to compare.

• Choosing the operation

  The arithmetic is fine, but the wrong operation was picked: adding when the story takes away, or multiplying when it shares out.

  Mistakes we catch: added instead of taking away and wrong operation.

  Try at home: Act out word problems with toys: “3 birds, 2 fly away.” Ask “are we putting together or taking away?” before any numbers.

• Reading the question

  The math works, but the answer is to a different question: a number already given, a halfway step, or only part of a two-step problem.

  Mistakes we catch: gave a number from the story.

  Try at home: Read the question aloud together and ask “what are we trying to find?” before solving. Point at the question part of the sentence.

More Grade 1 worlds: Numbers to 100 · Adding & Subtracting to 20 · Length, Time & Graphs · Shapes & Equal Shares · Equal Groups & Money