10 oranges + 20 grapefruits = 35 pounds
16 oranges + 10 grapefruits = 23 pounds
13 oranges + 15 grapefruits = ?
Let x represent one orange and y represent one grapefruit, you can express the given equivalencies as equations:
10 oranges + 20 grapefruits = 35 pounds → 10x+20y=35
16 oranges + 10 grapefruits = 23 pounds→ 16x+10y=23
13 oranges + 15 grapefruits = ? → 13x+15y= ?
With this, we determined an equation system with two unknowns.
Write the first equation in terms of y:
![\begin{gathered} 10x+20y=35 \\ 20y=35-10x \\ y=(35)/(20)-(10)/(20)x\text{ \rightarrow solve the divisions and simplify} \\ y=(7)/(4)-(1)/(2)x \end{gathered}]()
Next is to replace it in the second equation:

Third step is to calculate the value of y

Finally, now that you know the values of x and y you can replace the third formula and calculate:

13 oranges and 15 grapefruits will weight 32.3 pounds