Tricky maths questions

[PurpleDuck]

I did it a bit differently, by linking everyone to Ben's sister, through my original drawing of a square:

if S+3=B and Bx3=M and M+4=D, it means that (S+3)x3+4=D - just like a chain calculation in a 'number machine'.

Multiplying it out (BODMAS rules!), we get 3xS+9+4=D, so 3xS+13=D

if we know that 4xS=D, then 4xS = 3xS+13, which means that S must equal 13, so Ben's sister is 13, Ben is 16, his mum is 48 and the dad is 52.

[seriousmum]

Eureka!!!! Now I've got it

I was sharper four years ago with my DS and in a no CEM world!!

[PurpleDuck]

We practised quite a few 'number machine' type questions, they can be very useful and sometimes come up in the tests - including backwards, i.e. finding the initial number from the final answer of a multi-stage calculation.

[seriousmum]

You're right. I'm fairly certain it came out last year. Maybe they may decide to skip this year!

[Guest55]

PurpleDuck's choice of letters helps - there is no need to use x and y just because it's algebra! I always suggest 'sensible' letters but avoid 'p' [could look like a cost] or 's' [can be mistaken for a 5].

[PurpleDuck]

PurpleDuck's choice of letters helps - there is no need to use x and y just because it's algebra! I always suggest 'sensible' letters but avoid 'p' [could look like a cost] or 's' [can be mistaken for a 5].

Thank you, Guest55. I try to keep things as simple as possible, otherwise I will just confuse myself and DCs in the process... And yes, the way my eldest writes his '5' is such that sometimes he himself forgets whether it was to be a '5' or an 'S' - not very helpful when he is trying to solve some lengthy equations!

[seriousmum]

Great tip.

[don1976]

I appreciate I’m late the party here, but my daughter found it easiest to use substitution when we tried this when studying for the 11+, rather than selecting numbers to try.

F = M + 4
M = 3B

Therefore F = 3B + 4
B = S + 3

Therefore F = 3(S + 3) + 4 = 3S + 9 + 4 = 3S +13

F = 4S
Therefore 4S = 3S + 13

Taking the 3S to the other side we have 4S – 3S = 13 Therefore S = 13

From there, we know that B = S + 3 = 16

F = 4S = 4 x 13 = 52

M = 3B = 3 x 16 = 48