Two pieces of cake weigh as much as one apple and one cherry. One apple weighs as much as five cherries and one piece of cake. How many cherries weigh as much as one apple?

Answer 1

Let X = cake weight
Let Y = cherry weight
Let Z = apple weight
2X = Y + Z
Z = 5Y + X
Eliminate the variable X to get an equation between Y and Z.
2X = -10Y +2Z (from the second equation, multiplied by two and rearranged)
15Y -Z = 0
Z = 15 Y
15 cherries weight the same as an apple.