Answer:
You have to use 20 pounds of blackberries
Step-by-step explanation:
To start, we are going to make two equations, one that represents the amount of pounds and another that represents the prices
a = blueberries
m = blackberries
a + m = 30
a * 2$ + m * $3.50 = 30 * $3
let's clear in the first equation
a + m = 30
a = 30 - m
we replace in the second equation a by (30 - m)
a * $2 + m * $3.50 = 30 * $3
(30 - m) * 2 + m * 3.50 = 30 * 3
60 - 2m + 3.5m = 90
-2m + 3.5m = 90 - 60
1.5m = 30
m = 30/1.5
m = 20
we replace m with its value in the first equation
a = 30 - m
a = 30 - 20
a = 10