Question

use substitution to solve s+a=4 and 35+5a=2550

Answer 1

35+5(4-s)=2550
35+20-5s=2550
55-5s=2550
-55 -55
-5s = 2495
s = - 499

-499 + a = 4
+499 +499
a = 503

Answer 2

s + a = 4   

35 + 5a = 2550

From the second equation:

35 + 5a = 2550

5a = 2550 - 35

5a = 2515

a = 2515/5

a = 503


From      s + a = 4

s + 503 =4

s = 4 - 503

s = -499

Therefore  a = 503,  s = -499