diff --git a/subjects/macro_calculator/README.md b/subjects/macro_calculator/README.md
index be4d6d84..579845ee 100644
--- a/subjects/macro_calculator/README.md
+++ b/subjects/macro_calculator/README.md
@@ -2,22 +2,20 @@
 
 ### Instructions
 
-Create a **function** `calculate_macros` which receives a vector of `serde_json::Value` and returns a `serde_json::Value`.
+Create a **function** `calculate_macros` which receives a vector of `Food` structures and returns a `json::JsonValue`.
 
-The vector the function will receive will contain jsons in the following format:
-
-```json
-{
-    "name": <name>,
-    "calories": [<value_in_kJ>, <value_in_kcal>],
-    "fats": <fats_in_g>,
-    "carbs": <carbs_in_g>,
-    "proteins": <proteins_in_g>,
-    "nbr_of_portions": <portions>
+```rust
+Food {
+    name: <name>,
+    calories: [<value_in_kJ>, <value_in_kcal>],
+    fats: <fats_in_g>,
+    carbs: <carbs_in_g>,
+    proteins: <proteins_in_g>,
+    nbr_of_portions: <portions>
 }
 ```
 
-The name and the content of the calories array will be `strings`, all other values will be `floats`.
+Both values in the calories array will be `strings`, all other values will be `f64`s.
 
 The function should return a json with the following format (Macros struct):
 
@@ -32,13 +30,23 @@ The number of portions should be taken into account. The values of the macros re
 All values should be floats (f64) and should be the addition of every macronutrient in the provided array (cals is the addition of every calories, fats is the addition of every fats, and so on...).
 Every value should be rounded to two decimal places (ex: 12.29) or one decimal place if it ends in 0 (ex: 18.90 -> 18.9).
 
-Hint: You will need the `serde`, `serde_json` and `serde_derive` crates.
+Hint: You will need the `json` crate.
+
+### Expected Function
+
+```rust
+pub struct Food {
+    //expected public fields
+}
+
+pub fn calculate_macros(foods: Vec<Food>) -> json::JsonValue {
+
+}
+```
 
 ### Notions
 
-- [serde](https://crates.io/crates/serde)
-- [serde JSON](https://crates.io/crates/serde_json)
-- [serde_derive](https://crates.io/crates/serde_derive)
+- [json](https://docs.rs/json/0.12.4/json/) crate
 
 ### Usage
 
@@ -47,28 +55,27 @@ Here is a program to test your function:
 ```rust
 use macro_calculator::*;
 
-fn main() {
-    let a = serde_json::json!(
-        {
-        "name": "light milk",
-        "calories": ["148kJ", "35kcal"],
-        "protein": 3.5,
-        "fats": 0.1,
-        "carbs": 5.0,
-        "nbr_of_portions": 0.7
-    });
-
-    let b = serde_json::json!({
-        "name": "oat cookies",
-        "calories": ["1996kJ", "477kcal"],
-        "protein": 8.2,
-        "fats": 21.0,
-        "carbs": 60.4,
-        "nbr_of_portions": 1.2,
-    });
-
-    let macros: Macros = serde_json::from_value(calculate_macros(vec![a, b])).unwrap();
-    println!("{:?}", macros);
+fn main(){
+    let a = vec![
+        Food {
+            name: String::from("big mac"),
+            calories: ["2133.84kJ".to_string(), "510kcal".to_string()],
+            protein: 27.0,
+            fats: 26.0,
+            carbs: 41.0,
+            nbr_of_portions: 2.0,
+        },
+        Food {
+            name: "pizza margherita".to_string(),
+            calories: ["1500.59kJ".to_string(), "358.65kcal".to_string()],
+            protein: 13.89,
+            fats: 11.21,
+            carbs: 49.07,
+            nbr_of_portions: 4.9,
+        },
+    ];
+
+    println!("{:#}", calculate_macros(a));
 }
 ```
 
@@ -76,6 +83,11 @@ And its output:
 
 ```sh
 $ cargo run
-Macros { cals: 596.9, carbs: 75.98, proteins: 12.29, fats: 25.27 }
+{
+    "cals": 2777.39,
+    "carbs": 322.44,
+    "proteins": 122.06,
+    "fats": 106.93
+}
 $
 ```