№ 13 a) 6a + 6a = 636; б) 15b - 8b = 721; в) 9t + t = 7000; г) 10s - s = 540; д) t + 3t + 6t = 1800; е) 5t + 4t - t = 7200.

Решение:

1. а) 6a + 6a = 636
   \( 12a = 636 \)
   \( a = \frac{636}{12} \)
   \( a = 53 \)
2. б) 15b - 8b = 721
   \( 7b = 721 \)
   \( b = \frac{721}{7} \)
   \( b = 103 \)
3. в) 9t + t = 7000
   \( 10t = 7000 \)
   \( t = \frac{7000}{10} \)
   \( t = 700 \)
4. г) 10s - s = 540
   \( 9s = 540 \)
   \( s = \frac{540}{9} \)
   \( s = 60 \)
5. д) t + 3t + 6t = 1800
   \( 10t = 1800 \)
   \( t = \frac{1800}{10} \)
   \( t = 180 \)
6. е) 5t + 4t - t = 7200
   \( 8t = 7200 \)
   \( t = \frac{7200}{8} \)
   \( t = 900 \)

Ответ: а) a = 53; б) b = 103; в) t = 700; г) s = 60; д) t = 180; е) t = 900.