Sign up ×
Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

How many four coloured balls are there in a bag, such that all but $12$ are blue, all but $21$ are black, all but $20$ are red and all but $19$ are green

share|cite|improve this question

1 Answer 1

Hint: You have 5 equations and 5 variables: $$ x+12=T \\ y+21=T\\ z+20=T\\ a+19=T\\ x+y+z+a=T $$

Solve for T by plugging in the first four to the last.

share|cite|improve this answer
Last step can be thought of as adding the first four equations 'column-wise' in which case we have $T + 72 = 4T$. –  muzzlator Apr 2 '13 at 12:20

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.