diff --git a/src/fillTank.test.js b/src/fillTank.test.js
index 588d8e4..6eb0a29 100644
--- a/src/fillTank.test.js
+++ b/src/fillTank.test.js
@@ -1,11 +1,163 @@
 'use strict';
 
+const { fillTank } = require('./fillTank');
+
 describe('fillTank', () => {
-  // const { fillTank } = require('./fillTank');
+  let customer;
+
+  beforeEach(() => {
+    customer = {
+      money: 3000,
+      vehicle: {
+        maxTankCapacity: 40,
+        fuelRemains: 8,
+      },
+    };
+  });
+
+  it('should be declared', () => {
+    expect(fillTank).toBeInstanceOf(Function);
+  });
+
+  it('should not return anything', () => {
+    const result = fillTank(customer, 10, 20);
+
+    expect(result).toBeUndefined();
+  });
+
+  it(`should change the original 'customer' and not create copy`, () => {
+    const copy = JSON.parse(JSON.stringify(customer));
+
+    fillTank(customer);
 
-  it('should ', () => {
+    expect(customer).not.toEqual(copy);
+  });
+
+  it(`should full tank if 'amount' is not given`, () => {
+    fillTank(customer, 10);
+
+    expect(customer)
+      .toEqual({
+        money: 2680,
+        vehicle: {
+          fuelRemains: 40,
+          maxTankCapacity: 40,
+        },
+      });
+  });
+
+  it(`should full tank if 'amount' > 'tank capacity'`, () => {
+    fillTank(customer, 10, 48);
 
+    expect(customer)
+      .toEqual({
+        money: 2680,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 40,
+        },
+      });
   });
 
-  // write tests here
+  it(`pour not more fuel than client can buy`, () => {
+    customer.money = 300;
+    customer.vehicle.fuelRemains = 2;
+
+    fillTank(customer, 10, 43);
+
+    expect(customer)
+      .toEqual({
+        money: 0,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 32,
+        },
+      });
+  });
+
+  it(`pour not more fuel than client can buy,`
+    + `when 'amount' is not given`, () => {
+    customer.money = 300;
+    customer.vehicle.fuelRemains = 2;
+
+    fillTank(customer, 10);
+
+    expect(customer)
+      .toEqual({
+        money: 0,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 32,
+        },
+      });
+  });
+
+  it(`decline transaction if client wants to pour less than 2 liters`, () => {
+    fillTank(customer, 10, 1);
+
+    expect(customer)
+      .toEqual({
+        money: 3000,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 8,
+        },
+      });
+  });
+
+  it(`decline transaction if client cannot afford more than 2 liters`, () => {
+    customer.money = 10;
+
+    fillTank(customer, 10, 20);
+
+    expect(customer)
+      .toEqual({
+        money: 10,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 8,
+        },
+      });
+  });
+
+  it(`decline transaction if the tank can accomodate < 2L of fuel`, () => {
+    customer.vehicle.fuelRemains = 39;
+
+    fillTank(customer, 10, 20);
+
+    expect(customer)
+      .toEqual({
+        money: 3000,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 39,
+        },
+      });
+  });
+
+  it(`round fuel amount down to tenth`, () => {
+    fillTank(customer, 10, 5.56);
+
+    expect(customer)
+      .toEqual({
+        money: 2945,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 13.5,
+        },
+      });
+  });
+
+  it(`round the price to nearest hundredth`, () => {
+    fillTank(customer, 8.782, 20);
+
+    expect(customer)
+      .toEqual({
+        money: 2824.36,
+        vehicle: {
+          maxTankCapacity: 40,
+          fuelRemains: 28,
+        },
+      });
+  });
 });