Question 1178319
say a+b= 15
Then the sum of twice the first and thrice the second is 35
So, 2a+3b= 35

We have;
a+b=15 ---i
2a+3b=35 ---ii

Then a=15-b ---iii

Input a into eqn ii
2(15-b)+3b= 35
30-2b+3b= 35
Collect like terms
-2b+3b=35-30
b=5.

Input b into eqn i
a+5=15
a=15-5
a=10.  a=10, b=5.