diff --git a/README.md b/README.md
index 784e9c2..553d16c 100644
--- a/README.md
+++ b/README.md
@@ -4847,6 +4847,145 @@ func main() {
 Since the first iota is declared with the value `3` (` + 3`), the next one has the value `4`
 </b></details>
 
+<details>
+<summary>What will be the output of the following block of code?:
+
+```
+package main
+ 
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+ 
+func main() {
+	var wg sync.WaitGroup
+	
+	wg.Add(1)
+	go func() {
+		time.Sleep(time.Second * 2)
+		fmt.Println("1")
+		wg.Done()
+	}()
+
+	go func() {
+		fmt.Println("2")
+	}()
+
+	wg.Wait()
+	fmt.Println("3")
+}
+```
+</summary><br><b>
+
+Output: 2 1 3
+
+[Aritcle about sync/waitgroup](https://tutorialedge.net/golang/go-waitgroup-tutorial/)
+
+[Golang package sync](https://golang.org/pkg/sync/)
+</b></details>
+
+<details>
+<summary>What will be the output of the following block of code?:
+
+```
+package main
+
+import (
+	"fmt"
+)
+
+func mod1(a []int) {
+	for i := range a {
+		a[i] = 5
+	}
+	
+	fmt.Println("1:", a)
+}
+
+func mod2(a []int) {
+	a = append(a, 125) // !
+	
+	for i := range a {
+		a[i] = 5
+	}
+	
+	fmt.Println("2:", a)
+}
+
+func main() {
+	sl := []int{1, 2, 3, 4}
+	mod1(s1)
+	fmt.Println("1:", s1)
+
+	s2 := []int{1, 2, 3, 4}
+	mod2(s2)
+	fmt.Println("2:", s2)
+}
+```
+</summary><br><b>
+
+Output: <code><br>
+1 [5 5 5 5]<br>
+1 [5 5 5 5]<br>
+2 [5 5 5 5 5]<br>
+2 [1 2 3 4]<br>
+</code>
+
+In `mod1` a is link, and when we're using `a[i]`, we're changing `s1` value to. 
+But in `mod2`, `append` creats new slice, and we're changing only `a` value, not `s2`.
+
+[Aritcle about arrays](https://golangbot.com/arrays-and-slices/),
+[Blog post about `append`](https://blog.golang.org/slices)
+</b></details>
+
+<details>
+<summary>What will be the output of the following block of code?:
+
+```
+package main
+
+import (
+	"container/heap"
+	"fmt"
+)
+
+// An IntHeap is a min-heap of ints.
+type IntHeap []int
+
+func (h IntHeap) Len() int           { return len(h) }
+func (h IntHeap) Less(i, j int) bool { return h[i] < h[j] }
+func (h IntHeap) Swap(i, j int)      { h[i], h[j] = h[j], h[i] }
+
+func (h *IntHeap) Push(x interface{}) {
+	// Push and Pop use pointer receivers because they modify the slice's length,
+	// not just its contents.
+	*h = append(*h, x.(int))
+}
+
+func (h *IntHeap) Pop() interface{} {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[0 : n-1]
+	return x
+}
+
+func main() {
+	h := &IntHeap{4, 8, 3, 6}
+	heap.Init(h)
+	heap.Push(h, 7)
+  
+  fmt.Println((*h)[0])
+}
+```
+</summary><br><b>
+
+Output: 3
+
+[Golang container/heap package](https://golang.org/pkg/container/heap/)
+</b></details>
 ## Mongo
 
 <a name="mongo-beginner"></a>