What single number can be added to both the numerator and the denominator of the fraction 3/7 so that the new fraction becomes equal to 2/3?
5
Let x be the number. (3 + x) / (7 + x) = 2/3. Cross multiply: 3(3 + x) = 2(7 + x) => 9 + 3x = 14 + 2x => x = 5.
About this riddle
Math riddles hide a calculation inside a short story. Pull out the numbers, work out what is being asked, and watch for a trick such as a unit change.
Difficulty: Hard. More like this: Math riddles with answers or try Target 24 for a different kind of puzzle.
More math riddles
Roses cost 2 dollars each. How much do 7 roses cost?
I am a two-digit number. If you reverse my digits, my value increases by 54. The sum of my two digits is 10. What number am I?
When flipping a fair coin three times in a row, what is the probability of getting at least one head? Express your answer as a simplified fraction.
Four people need to cross a dark bridge with one flashlight. At most two can cross together, walking at the pace of the slower person. Person A takes 1 minute, B takes 2 minutes, C takes 7 minutes, and D takes 10 minutes. What is the minimum time in minutes for everyone to cross?
If you multiply my age by itself, subtract 40, and then divide by 4, the result is exactly 9. How old am I?
Sam has 11 pencils and Ana has 6. How many more pencils does Sam have?