Respuesta :
They can all be used.
A. a – 42 = 91
+42 +42
--------------
a= 133
B. 42 + a = 91
-42 -42
____________
a= 49
C. 91 + a = 42
-91 -91
-----------------
a= - 49
D. a – 91 = 42
+91 +91
---------------
a= 133
A. a – 42 = 91
+42 +42
--------------
a= 133
B. 42 + a = 91
-42 -42
____________
a= 49
C. 91 + a = 42
-91 -91
-----------------
a= - 49
D. a – 91 = 42
+91 +91
---------------
a= 133