Question 1015916
B+R=280

2/3B=1/2R

Multiply both side of eq 2 by 6 (the least common multiple)

4B=3R

Rearrange eq 1 to   R=280-B

Now substitute

4B = 3(280-B)

4B = 840 - 3B

7B = 840 

B = 120

R = 160

It adds to 280, and when he used 1/3 of B, 80 are left, the same as 1/2 of R. Done!