The Baker Street Math Crossword
Dr Watson has brought Sherlock Holmes a page of sums, and some of the numbers are missing. The Baker Street Math Crossword is a grid of number sentences that read across and down. Most numbers sit in one line each way, so a number that fits one line has to fit the one that crosses it as well. Fill in the blanks until every line is true, and the case closes by itself.
There are three ranks. A Constable, for Grades 1–2, adds and takes away within 20 on a grid of nine numbers, with four blanks to find. An Inspector, for Grades 3–4, either adds and takes away up to 100 or uses the times and divide facts up to 10 × 10, with five blanks. A Detective, for Grades 5–6, gets a bigger grid with three numbers in each line, such as 3 + 4 × 5 = 23, so times and divide come before add and take away, and has six blanks to find.
Every case can be solved by steps you can be sure of: somewhere there is a line with just one blank, and only one number fits it. If you are stuck, Ask Holmes fills in one blank and says which line it used. Five cases make a casebook worth up to 15 magnifying glasses. The page remembers the rank you chose last, and your best casebook for each rank is saved on this device.
Frequently asked questions
How do you play the Baker Street Math Crossword?
Tap a blank, or tab to it, and type the number; on a phone the number keypad opens, and a blank takes up to three digits. The arrow keys move between blanks. A Constable and an Inspector also have a bank of the missing numbers: tap one and it goes into the selected blank and is struck off the bank. There is no submit button, because the case closes by itself when the last blank is right. Check marks every wrong number, and Ask Holmes fills in one blank and says which line it used.
How can you solve a case without guessing?
Look for a line with only one blank. With the other numbers in place, just one number can make the line true, so you can work it out. Say Down 2 reads 6 × ? = 42: only 7 works, because 6 × 7 = 42. That 7 also sits in a line across, which may now have only one blank left, and so on, one line at a time. Every case in the game can be finished this way, so none needs a guess. If a number makes one line true but the line that crosses it false, one of the two lines has a wrong number in it.
What are the Constable, Inspector and Detective ranks?
A Constable case is a grid of nine numbers in three lines across and three down, each line adding or taking away, with no number over 20 and four blanks. An Inspector case is the same grid with five blanks, and it is either all adding and taking away with numbers up to 100, or all times and divide with factors from 2 to 10 and nothing over 100. A Detective case has four numbers in each line, the last being the result, 15 numbers in all and six blanks. Each line has one times or divide and one add or take away, such as 3 + 4 × 5 = 23, because 4 × 5 = 20 comes first and 3 + 20 = 23. The numbers in the middle are from 1 to 12 and no result is over 150.
How are the magnifying glasses scored?
Each case is worth up to 3 magnifying glasses. Asking Holmes costs one, and so does a Check that finds a wrong number. A Check that finds nothing wrong costs nothing, and so does a wrong number you fix before you Check. A case you solve always earns at least 1, so a case is worth 1, 2 or 3. Five cases make a casebook, worth 15 at most and 5 at least. Your best casebook is kept for each rank, on this device only.
What does the number bank do?
The bank lists the missing numbers, smallest first, and strikes one off as you place it. A number that is missing twice appears twice, and is struck off twice. It lets a younger player try a number in a blank and see whether both lines accept it. A Detective works without a bank, so the missing numbers have to be worked out from the lines alone.