Mark has some yellow and black marbles. The total mass of all his marbles is 245 g. He has 7 less yellow marbles than black marbles. The average mass of the yellow marbles is 3 g. The average mass of the black marbles is 4 g. How many marbles does Mark have altogether?
| |
Yellow |
Black |
Total |
| Number |
1 u |
1 u + 7 |
2 u + 7 |
| Value |
3 |
4 |
|
| Total value |
3 u |
4 u + 28 |
245 |
Total mass of yellow marbles
= 1 u x 3
= 3 u
Total mass of black marbles
= 4(1 u + 7)
= 4 u + 28
Total mass of the marbles
= 3 u + 4 u + 28
= 7 u + 28
7 u + 28 = 245
7 u = 245 - 28
7 u = 217
1 u = 217 ÷ 7 = 31
Total number of marbles
= 2 u + 7
= 2 x 31 + 7
= 62 + 7
= 28
Answer(s): 28
