On average, 2 will fall apart. We can tell this by making a proportion.
1/20 = x/40 ---> cross multiply
1*40 = 20*x ---> multiply
40 = 20x ----> divide by 20
2 = x
The number of situps form an AP with first term (a) = 17 and common difference (d) = 4
Tn = a + (n - 1)d
T12 = 17 + (12 - 1) x 4 = 17 + 11(4) = 17 + 44 = 61
Therefore, she will do 61 situps on day 12.
Let's see all the steps:
1) operation : 5 options
2) only one option: we ignore it
3) the third operation: 6 options
4): 2 options.
So we have 5 options, 6 options and 2 options: and they are independent of each other. This means that we multiply the numbers, and 5*6*2=60 So the answer is 60!
