Please help. Simple but not for me

[Jo2362]

The question is how would you tackle a question of this sort please?

I have spent £38 in coconuts and melons, the coconuts are £5 each and the melons £7. How many melons and have I bought? Obviously I know that the answer is 4, however, I would very much appreciate it if anyone could point out what the formula/approach would be to tackle this sort of question.

Thanks in advance

Ps. I am new so please be kind, if I have posted in the wrong place I am sorry.

Jos

 

[JohnZ]

The question is how would you tackle a question of this sort please?

I have spent £38 in coconuts and melons, the coconuts are £5 each and the melons £7. How many melons and have I bought? Obviously I know that the answer is 4, however, I would very much appreciate it if anyone could point out what the formula/approach would be to tackle this sort of question.

Thanks in advance

Ps. I am new so please be kind, if I have posted in the wrong place I am sorry.

let C be the number of coconuts, M be the number of melons
5*C+7*M=38
then try C=1,2,3...

 

[david_the_tutor]

Coconuts & melons

The question is how would you tackle a question of this sort please?

I have spent £38 in coconuts and melons, the coconuts are £5 each and the melons £7. How many melons and have I bought? Obviously I know that the answer is 4, however, I would very much appreciate it if anyone could point out what the formula/approach would be to tackle this sort of question.

Thanks in advance

Ps. I am new so please be kind, if I have posted in the wrong place I am sorry.

Jos

Let the number of coconuts and melons purchased, be C and M, respectively
Therefore, 5C + 7M = 38
5C = 38 - 7M
C = (38 - 7M)/5 ------- eq (i)

C, or number of coconuts has an INTEGER VALUE, and, as seen above, this integer value is also a MULTIPLE of 5
Based on eq (i), 38 - 7M CANNOT BE NEGATIVE, so M CANNOT be > 5, since an INTEGER value > 5 will result in a negative value for the numerator, 38 - 7M

Thus, we try digits ≤ 5 for M
M = 5
C = (38 - 7M)/5
C = [38 - 7(5)]/5
C = (38 - 35)/5_____This value DOES NOT REPRESENT an INTEGER VALUE for the number of coconuts, or C

M = 4
C = (38 - 7M)/5
C = [38 - 7(4)]/5
C = (38 - 28)/5_____This value DOES REPRESENT an INTEGER VALUE for the number of coconuts (C)
C = 10/5, or 2

Therefore, 4 melons and 2 coconuts were purchased.

You can do the check!!