Set up a table with ingredient, cost per l., no of l.,and total cost
Total cost is amount of l * cost per l.
green mango 4.25/l x 4.25x
yellow mango 6.5/l 150 6.5/l( 150)
mixture 5/l x+150 5( x + 150)
In the second column, the total amount of the mixture is the amount of green mango plus the amount of yellow mango.
Your equation comes from adding up the last column: the total cost of the green + total cost of yellow =total cost of the mixture
4.25 x + 6.5 (150) = 5( x + 150)
Solve from there.
Hope this helped.